Grounding and Electrical Safety of
Shore Power Supply and DC Stinger System Design
Dev Paul
AECOM
2101 Webster Street, Suite 1900, Oakland, Ca 94612
dev.paul@aecom.com; (510) 419 – 6448
Abstract: To enhance personnel safety and equipment protection, design of the shore power supply,
also known as “Head End Power” (HEP) to the diesel locomotive train should consider appropriate
source grounding design. Statistical data indicate that 75% to 85% of all faults involve ground; and
therefore design of the power system source grounding should be appropriately designed to minimize
ground fault current and associated equipment damage and unpredictable threat of safety to the
maintenance person. The first part of this paper explores this problem.
The second part of the paper will cover design of dc power supply to the maintenance shop required for
maintenance of the vehicles that use dc power supply for train operation and vehicle auxiliaries. This
paper will cover both types of vehicles that receive power from the overhead contact wire system (OCS)
and from the 3rd rail system. Hazards of dc arc during fault condition are well known to the transit
industry, however, author believe that there is a need for APTA/IEEE recommended practice for the
design of dc power supply inside the maintenance shop. This will allow UL listed enclosures for the 1000
V dc control panels, 1000V dc disconnect switches, and local controllers for the contactor’s and stingers.
Finally, this paper provides recommendations for the HEP ac power supply and the DC power supply in
the maintenance shop.
Index Terms – Arcing fault, disconnect switch, fuse, grounding, head end power (HEP), ground fault,
overhead contact wire system (OCS), stinger system, maintenance shop, protection, system charging
current, and tripping.
I.
INTRODUCTION
HEP power supply at 480V, 3-phase to the vehicle should consider: what is the impact of ground
fault developed by wiring system inside the vehicle? It appears that even if the wiring terminations
at equipment inside the vehicle and inside the enclosures use appropriate terminations for the
vehicle operation, however, the vehicle movement has certain effect on the terminations making
them loose or simply cause some fatigue. Ground fault current can be minimized by appropriate
design techniques of controlling the ground fault current by resistance grounded system, but they
can’t be eliminated due to inherent characteristics of the power system. Ground fault damage is
1
related to the magnitude of the ground fault current and it’s clearing time. One method of reducing
the ground fault current is to use a neutral resistor at the power source. By design experience,
industry learned that the minimum ground fault current needs to be slightly higher than the system
charging current in order to prevent transient overvoltage on the un-faulted phases during lineground fault on one phase [1]. This type of resistor grounding is called High-Resistance Grounding
(HRG) system [1] [10] [13] [14] [16].
By application of a sensitive toroidal neutral current transformer (CT) and a sensitive micro-process
ground fault relay, device 50/51G, any ground fault with fault resistance (fault current as low as
20% of the bolted line-ground fault) can be isolated in minimum time to minimize the damage due
to ground fault and to improve personnel safety and minimize equipment damage. More recent
developments of the new standard [2] on the electrical arc-hazards and enforcement of OSHA
safety regulations; it has become extremely important to review current recommended APTA
guidelines. These APTA guidelines are APTA PR-E-RP-016-99 [3], and APTA PR-E-RP-015-99 [4], and
PRIIA 305-009/Amtrak 995 Technical Specifications (Chapter 13) [5]. Other related codes and
standards that provide recommendations for improvement in grounding, ground fault protection
and to minimize equipment damage due to ground faults are IEEE standard [1] and IEEE technical
papers [10][13] [14][16]. This paper makes recommendation for using HRG grounded power source
for design of HEP power supply based upon ground fault current analysis affecting equipment
damage and personnel safety. Existing APTA standards should be reviewed to make HEP power
from a HRG grounded power source. For such a design, the design engineers, contractors, rail
transit authority, supplier of HEP controller, and vehicle supplier all need to know the benefits of
HRG system and agree to such a design recommendation.
In the maintenance facility 750V dc or 1000V dc equipment enclosures can cause safety concerns, if
the maintenance staff has not taken electrical arc hazard training along with management staff at
the rail transit facility. No recommended guidelines exist today to perform dc arc hazard analysis
[11] similar to IEEE Standard 1584 for AC arc hazard [2]. Therefore safety of dc arc hazards, dc
electrical equipment energy labeling and warning signs, and importance of using personnel
protective equipment (PPE) can’t be enforced in a scientific manner.
For safety reasons, this paper presents recommendation for HEP power system to be HRG
grounded. Paper also makes recommendation for APTA recommended guidelines for dc power
supply to maintenance shop.
II.
HEAD END POWER (HEP)
A brief description of shore power supply (HEP power supply) for the diesel locomotive passenger
rail cars (vehicles) is included in this section. Power supply for the vehicle lighting, HVAC
equipment, PA system, CCTV, dining facilities, door open close mechanism including indicating
lights, and operation of some emergency push buttons are operated from low voltage ac or dc. This
power supply is generated by on-board diesel-engine generator rated at 480Y/277V voltage and
2
then stepped down for 120V. In the maintenance facility diesel locomotive automatically shutdown diesel engine propulsion system along with auxiliary power generator. Then this three-phase,
480V, 3-wire, 60 Hz power supply is distributed from a shore-side power controller by using three
power conductors and three control wires to the first car, which then get distributed to each car in
a train by use of a standard flexible cable and couplers between each car [4].
To minimize ground fault hazard within the cars, normally electrical generator is equipped with
HRG grounding and ground fault protection scheme or it is ungrounded system. However, author
recommends not using ungrounded power system discussed in section III of this paper. For
maintenance and routine check of the vehicles, the power from the utility source is provided by use
of plug and receptacle assembly to first vehicle, also known as stinger system. To minimize the
ground fault hazard and to keep touch potential of the vehicle low (safe voltage less than 30 V)
during line-ground fault, the shore power supply should use HRG grounded isolation transformer.
Since the utility power supply for the maintenance building is generally supplied by the utility
solidly grounded transformer and thus the use of an isolation transformer with 480V delta primary
to 480Y/277V wye secondary with HRG grounded neutral is necessary.
Current APTA standard APTA PR-E-RP-015-99 [3] needs to be updated to make it clear that the
upper limit of ground fault indicated as 1 A. This current is based upon system charging current of
the 480 V HEP power system including vehicle wiring systems not to exceed 1 A. This value of fault
current not possible unless it is from a HRG system which controls the line-ground fault current.
This paper is considering this value to be 0.5 A as 480V HEP wiring charging current will not be
more 0.5 A, especially if the trains are two cars or three cars. However, whether it is 1A or 0.5A
current, it is only possible if it is from a power source using HRG grounding scheme. It should be
noted that the return current path for the ground fault current for a fault inside the vehicle can
vary from season to season, very low during rainy season and very high during dry weather season.
Other relevant sections of this standard describing solidly grounded power source should be
changed to HRG grounded power source. Likewise, other APTA standards [4] and [5] also require
proper review to reflect HEP power supply from a HRG grounded power system.
III. CHARACTERISTICS OF HIGH RESISTANCE GROUNDED (HRG) POWER SOURCE
When the resistor value at the neutral of the power source is such that during bolted line-ground
fault on one phase, the maximum current flow through the neutral resistor is equal to or slightly
greater than the system charging current then it is called high-resistance grounded (HRG) power
system [1]. Such a HRG power system is not new to the industry; it is applied to both the medium
voltage (MV) and to the low voltage (LV) power system design to many industries. Presently, for
petrochemical, refineries, and some other critical process industries, LV with HRG system is applied
to continue operation upon detecting first line to ground fault with alarm. This allows operator to
orderly manage the ground fault condition without shutting down the system and impacting the
critical plant operation. In addition, arc hazard analysis proved this system to be much safer
compared to solidly grounded power system.
3
This paper will first provide a brief description of the design criteria of a HRG system, then the
ground fault current flow diagram during bolted line-ground fault condition which has been
misunderstood by the industry and need correction to IEEE Std. 142 [1][16]. A discussion of a
practical line-to-ground arcing fault; assuming impedance of the arc as pure resistance is described
in a published paper, “A Novel Method of Measuring Inherent Power System Charging Current”
[16]. In this paper, a vector diagram was used to illustrate the effect of arc resistance on a lineground fault current, damage at the fault location, and concluding that the HRG criteria will be
maintained during arcing ground fault condition. Using vector diagram approach, this paper
provided an electrical equivalent circuit of a HRG system during line-to-ground arcing fault
condition assuming arc impedance as a pure resistance. Follow up papers [13] [14[16] by the same
author, used analytical approach to describe the same equivalent circuit establishing much needed
technical knowledge about the characteristics of HRG system. This equivalent circuit helps to
understand quickly how HRG criteria is maintained, how arcing fault can lead to damage at the
fault location and mathematical relationship between the arc resistance and the neutral resistor.
A.
Inherent System Charging Current
Each power system component is inherently coupled to ground (earth) through certain
capacitance. This capacitance value varies depending upon the type of equipment, for examples
cables has more inherent capacitance to ground as compared to switchgear. This inherent
capacitance appear as distributed equal capacitance between each phase to ground and therefore
inherent system charging current of equal magnitude is supplied by the power system at all times,
once the power system is energized.
Fig. 1 HRG Grounded power system, normal condition, system charging current flow
4
If the capacitance reactance of each phase to ground is assumed as XCO; which is equal in magnitude
for practical purposes, then the inherent system charging current (ICO) of each phase will be shown
by equation (1).
𝐼𝐶𝑂 = 𝐸𝐿𝑁 /𝑋𝐶𝑂
(1)
Under normal operation, the vector sum of the individual phases charging current will be zero as
they are at 120 degree phase apart from each other. Although such a system charging current is
flowing in each phase, but no current flows through ground and back to power source. Distributed
capacitance poses balanced 3-phase load on the power system.
For analysis purposes, inherent distributed capacitance of three phase power system can be
considered as a lumped wye grounded capacitor bank on the system. Irrespective of whether the
system is grounded or ungrounded inherent, system charging current will always be present so
long the power system is energized. Total system charging current (IC) will be as follows:
𝐼𝐶 = 3𝐼𝐶𝑂
(2)
Fig. 2 HRG grounded power system, line to ground bolted fault on phase C, (a) fault current flow
diagram (b) system charging current and neutral resistance current flow (c) vector diagram of
the ground fault current.
B.
Design Criteria for a HRG System
5
The magnitude of the total inherent system charging current (IC) should be known to design a HRG
power system]. Estimation of the system charging current can be determined based upon the
power distribution system equipment configuration as described in reference paper, “charging
current data for guess-free design of high-resistance grounded systems” [8]
Discussion of a bolted line to ground fault in the HRG power system is a good example for
understanding the maximum expected ground fault current at the fault location and two
components of the fault current i.e. resistive current IR and capacitive current IC as shown in Fig. 2.
If the system charging current is known, then neutral grounding resistor RN rating in ohms can be
determined. The criteria for the HRG system is that during bolted line-ground fault condition on
one phase the following relationship should apply [1]:
𝐼𝑅 ≥ 𝐼𝐶
(3)
⃗⃗⃗
⃗⃗⃗
𝐼⃗⃗⃗⃗⃗
𝐺𝐹 = 𝐼𝑅 + 𝐼𝐶
(4)
𝐼𝐺𝐹 = √𝐼𝑅 2 + 𝐼𝐶 2
(5)
𝑁𝑅𝐺 (𝑅) ≤
𝐸𝐿𝑁
⁄(𝑆𝐼 ) Ω
𝐶
(6)
Where:
NRG (RN)
Neutral grounding resistor in ohms
𝐸𝐿𝑁
Each phase line to neutral voltage in volts
𝐼𝐶
Inherent system charging current in amperes
𝐼𝑅
Current through RN in amperes
𝐼𝐺𝐹
Line to ground fault current in amperes
S
Safety factor, author recommends this to be 1.25 to account for an error in estimating
system charging current based upon the engineering calculations using distributed
capacitance of every component of the power system to ground.
This criteria is based upon the experience that during line to ground arcing fault condition on one
phase of a HRG system, the transient overvoltage on the other two phases may lead to 5 to 7 times
the rated system voltage (line-to neutral 60 Hz) within few cycles if the resistive component of the
fault current becomes lesser than the system charging current, as it will act as ungrounded power
system. To avoid this transient overvoltage condition, NGR rating should be selected to account for
the inaccuracy of estimating the system charging current. [16].
For this reason, some safety factor (S) as indicated in equation (6) should be applied to increase IR
to be more than IC during bolted line to ground fault current condition.
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C.
Damage at the fault location
The damage at the fault location is related to equation (7) below.
Damage = IGF 2 (t) RF dt (Watt-seconds) Joules
(7)
Where the magnitudes of current (IGF), fault resistance (RF), and the fault clearing time (t) are in
amps, ohms, and seconds respectively. Some researchers believe that the exponent 2 of IGF in
equation (7) should be 1.5. Whether it is 2 or 1.5 basic technical characteristics of the expression
and proportional damage remain the same. From equation (7) it is clear that damage will be least
based upon the optimized combination of three parameters, IGF, RF, and t. Parameter RF is
unpredictable and can’t be controlled, and this fault arc resistance (resistance of foreign element in
the path of the fault current flow circuit) has the effect on fault current and damage at fault
location. Referenced paper [16] provides comprehensive information about damage at fault
location for an HRG power system and it applies to both the LV and MV power system. It is clear
that by minimizing the fault current by HRG design and then clearing the fault without any
intentional time delay to minimize damage at fault location. This sets the criteria of using HEP
power from an HRG grounded power system.
IV. HEP POWER SUPPLY DESIGN TO DIESEL LOCOMOTIVE VEHICLE
Recommendations for the HEP design to the vehicle are to enhance personnel safety and to
minimize damage to equipment in case of line-ground fault hazard. Fig. 3 shows 480 V power
supply to HEP panels. Later discussion of power system grounding will conclude that we need HRG
grounded system as shown in this diagram.
The following information is taken from various published documents to establish characteristics of
480V, 3-phase, and 60 Hz HEP power supply.
A.
APTA PR-E-016-99 Edited 3-22-04
Section 4.4 Single Bus HEP, indicates 480V, 3-phase, 3-wire, ungrounded system, 60 Hz operation
Section 4.5 Split Bus HEP, indicates 480V, 3-phase, 3-wire, ungrounded system, 60 Hz operation
Under both sections a note in brackets clarifies characteristics of power supply source and load as
follows: “The source may have a neutral ground reference, but the distribution system and load do
not”. This clearly indicates that the power system is grounded as its neutral is referenced to
ground. Therefore this statement needs some modification.
Section 8.1 describes load type as 480V, 3-phase, ungrounded, 60 Hz. This statement is technically
incorrect as there are no loads today in the power system design that are grounded, and thus this
statement needs correction.
B.
APTA PR-E-015-99 Edited 3-22-04
7
Section 4.2.1 Single Bus HEP, indicates 480V, 3-phase, 3-wire, ungrounded system, 60 Hz operation
Section 4.2.2 Split Bus HEP, indicates 480V, 3-phase, 3-wire, ungrounded system, 60 Hz operation
Under both sections a note in brackets clarifies characteristics of power supply source and load as
follows: “The source may have a neutral ground reference, but the distribution system and load do
not”. This clearly indicates that the power system is grounded as its neutral is referenced to
ground. Therefore this statement needs some modification.
Section 5.1 2 HEP alternator and regulator clearly indicates machine should be high impedance
grounded (consistent with the ground fault detection requirements of 8.1.2 and 8.2.2 to the
locomotive car body.
Section 5.3.2 and Section 5.4.1 Inverter: both indicate that the unit should be connected through
high impedance grounded to the locomotive car body.
Section 5.5.1.2 Indicator: require ground fault indicator
Section 5.5.2 Protection: require indicator only, no power shutdown
Section 6.2.1 Utility Source: indicate transformer should have a grounded wye connected
secondary. No further clarification on the impedance grounding of the transformer.
Section 6.3.3 Protection: utility source HEP and Section 6.3.3 Protection: engine-driven source HEP,
both require ground fault protection
Section 7.5 Wayside power commission tests require calibration of the ground fault trip setting
Figure 7 [3] Single line Schematics HEP Sources indicate impedance grounded power sources
Figure 10 [3] Power Schematic: 480V wayside HEP indicate 480Y/277V with neutral conductor (all
other documents indicate without neutral).
C.
PRIIA 305 -009/Amtrak 995 Technical Specifications (Chapter 13)
At no place in this document provides characteristics of 480V HEP power supply other than in
Section 13.4.2 Wiring and Connections it indicates wire 4/0 recommended in APTA recommended
Practice RP-E-016-99.
Based upon the reviews of referenced documents [3] [4] [5] described earlier in this paper, it
should be concluded that the documents try to describe that the HEP power should result in
minimum ground fault current equal to system charging current of ungrounded power system
(equal to system charging current) or square of two of the system charging current in case of high
impedance grounded system during bolted line to ground fault current. Since this ground fault
current is low and thus it may be concluded that there is no need to trip the power source but an
alarm of the ground fault condition is needed to isolate the faulted section of the power system in
an orderly fashion. To trip or not to trip automatically upon detecting first line-ground fault should
8
be the decision by the car supplier. It has been determined that on an ungrounded power system
although the line-ground fault is very low, however, if the fault is an arcing fault it will result in a
very high transient overvoltage 5 to 7 times normal peak voltage in very short period of time
(within 5 to 6 cycles) which can result in insulation failure and destruction inside the car at the fault
location [1]. From this it is concluded that ungrounded power system is not suitable. It is
recommended that a high-resistance grounded (HRG) power system with automatic ground fault
tracing and alarm should be used.
D.
480V Power Supply System Grounding choice based upon safety analysis
It is an important element of the design that all parties involved in the design construction and
operation (designer, contractor, transit facility personnel, vehicle supplier, and supplier of HEP
controller) should understand this important issue of using solidly grounded or ungrounded power
source. This will result in proper HEP power controller by the manufacturer with proper protection
features. It is important that the vehicle supplier also provide proper necessary information for the
on-board 480V power supply system to assure it matches with the existing on-board ungrounded
or high impedance grounded power system. Author does not recommend ungrounded vehicle onboard system as it can cause insulation damage or even fire.
In one project, author was involved in investigation of the HEP power supply and indicated that an
isolation 480 delta to 480Y/277V wye with high resistance grounded neutral should be used to
supply HEP power supply to the vehicle instead of using solidly grounded 480Y/277V power supply
tapped from the maintenance facility power provided by the solidly grounded utility power
transformer. Safety analysis of using HRG system and solidly grounded system are discussed in this
paper.
In the examples below, a bolted line-ground fault is used which provides highest ground fault
current in each type of grounding system, solidly grounded or HRG grounded.
E.
Solidly Grounded Power System
It is noted that a line-ground fault current in case of solidly grounded power system will be equal to
or higher than the three phase fault current depending upon the impedance of the zero sequence
path to be equal to lower than the positive or negative sequence impedances. Generally positive
sequence impedance is equal to negative sequence impedance. Thus; it may cause major ground
fault current leading to dangerous vehicle touch potential and may even cause damage to vehicle
equipment at the fault location, see Fig. 4 and Fig. 5. Person in contact with the vehicle will have
current flow derived from the equivalent circuit Fig. 5 as follows:
ISHORT = 277V/10Ω = 27.7A
(8)
IBODY = ISHORT [0.25/ (0.25+500)] = [(27.7 x 1000) x (0.25/500.25)] = 13.84 ma
(9)
Touch potential for the above body current will be approximately 277 V assuming negligible voltage
drop in the feeder cable.
9
F.
HRG Grounded Power System
Fig. 6 and Fig. 7 show bolted line to ground fault current flow and ground fault equivalent circuit
respectively for a HRG grounded power system.
ISHORT = 277V/564Ω = 0.491 A
(10)
IBODY = ISHORT [0.25/ (0.25+500)] = 0.245 ma
(11)
The touch potential for the above calculated body current will be approximately 277 V, assuming
voltage drop in the supply feeder is negligible compared to voltage drop in the HRG resistor.
It should be noted that the body current is case of HRG system is very less compared to solidly
grounded power system (0.245 ma compared to 13.84 ma). This current may not be dangerous to
the maintenance person or damaging to car at the fault location so long it is sensed and fault is
isolated without intentional time delay.
It must be emphasized that if without much understanding of the equipment grounding design
where design engineers and contractors normally provide underground 4/0 bare copper to bond all
electrical equipment enclosures and then make connection to running rails. This obviously conclude
that the at both the running rail and at the transformer 5 ohms grounding electrodes shown in Fig.
4 will be bypassed and then the fault current and body currents will change as follows.
ISHORT = 277V/0.30Ω = 923A
(10A)
IBODY = ISHORT [0.25/ (0.25+500)] = [(923 x 1000) x (0.25/500.25)] = 462.00 ma
(11A)
The above calculation is based upon assumed total impedance of ground fault current loop of 0.30
ohms, however, return path impedance through ground can vary with season. This current may not
be sensed by the feeder breaker to clear the fault and thus may present danger. It may also cause
damage to car body at fault location if fault persists for longer duration without tripping the power
system.
Recommendation is that no grounding and bonding of rails to electrical enclosures be allowed for
HEP power supply. This applies equally to HRG grounded power system.
Therefore HRG system which controls maximum fault current to be equal to square root of two
times the system charging current in case of a bolted phase to ground faults condition. The body
current is not dangerous due to limited line-ground fault current and damage to vehicle is also less
as the fault current is low. If the system do not need to be tripped on a first line to ground fault on
a HRG system and alarm is adequate then it will also work as orderly isolation of the faulted section
can be accomplished. In addition, HRG systems today comes with automatic controller for tracing
of ground fault location. In North America all petro-chemical pants and refineries use HRG system
with alarm and automatic fault location. This provides continuous operation of the critical
operation with minimum danger of arc hazard and minimum equipment damage.
10
Based upon these considerations, locomotive vehicles for DMU trains should use HEP power from
HRG grounded power system with proper ground fault sensor for alarm and automatic delayed
tripping if so desired.
Fig. 3 One line diagram of power supply to a maintenance facility including HEP power supply
11
Fig. 4 solidly grounded power supply system – fault current flow during ground fault
condition on phase C inside the vehicle
Fig. 5. Equivalent circuit for a ground fault condition shown in Fig. 4
12
Fig. 6 HRG grounded power supply system – fault current flow during ground fault
condition on phase C inside the vehicle
Fig. 7. Equivalent circuit for a ground fault condition shown in Fig. 6
13
G.
Method of HEP power supply connection to vehicle
Industry have established standards for HEP power connection using power supply cabinet located
on wayside which will be connected to shore power supply and then inside the cabinet will be a
contactor which will establish 480V power by closing contactor only when train-line connection
(TLC) interlocks establish that all power and control connectors from one vehicle to next vehicle are
established.
Fig. 8 below depicts such an interlock assuming dc control power supply is inside the vehicle and
HEP power supply cabinet will use this control power supply for design interlocks. One conceptual
design of HEP power supply controller is shown in Fig. 9. Figure 10 show all control details for the
HEP cabinet with the exception that the supplier of this cabinet has not been informed that the
maximum ground fault anywhere on the HEP system will never be more than 1A as has been
described in APTA standard [4] and the thus designer of the shore power supply will use 480V to
480Y/277V isolation transformer with HRG scheme. Therefore, ground fault CTs ratios needs to be
low (5/1A ratio) instead of 200/5 A CTs shown in this Fig. 10. High accuracy CTs are recommended
inside the HEP cabinet for sensing low ground fault current much less than 1 A.
Fig. 8 Train-line complete (TLC) loop system
Fig. 9 Typical vehicle supplier TLC loop and HEP power supply scheme
14
Fig. 10 HEP power supply controller schematic diagram
V.
MAINTENANCE FACILITY - DC POWER SUPPLY SYSTEM TO LRT SYSTEM
This section of the OCS within the shop maintenance building is electrically isolated by inline
section insulator. In addition, the shop building gate located at the entrance tracks automatically
opens when vehicle enters the shop in such way that it bridges the OCS contact wire with an inline
insulator when door is in open position. The vehicle entering the shop will have its pantograph
down and the OCS Bridge is down for the gate to open and then when vehicle enters the building
the OCS Bridge automatically lifts up to bridge the OCS contact wire. Designs of such an operation
require proper interlocks of all interfacing devices. The main objective is to keep inside OCS section
electrically isolated/insulated from the outside section of the OCS.
There are two separate designs to provide dc power to the maintenance vehicle, they are by a) use
of OCS and B) use plug and receptacle assembly also called stinger system. They are described
below.
15
A.
OCS Supply System
Fig. 11 and Fig. 13 show one line diagram and control schematic respectively for the shop dc power
supply to shop OCS system. Proper layout design of the OCS wire near the ceiling and the monorail
operation in the close vicinity to the OCS is coordinated so that their operation is not affected and
poses any hazard. Fig. 14 show wall mounted dc disconnect switch, ANSI/IEEE device (89), which is
electrically interlocked with a companion dc disconnect switch, ANSI/IEEE device (89),located inside
the dc switchboard by use of a 125V dc contactor. This 125V dc contactor, ANSI/IEEE device (29),
inside the enclosure becomes local controller for energizing and de-energizing the OCS. Interposing
control relays and indication lights can be used so that contactor operation is safe from switching in
the dc contactor contact in a no load condition for establishing the 750V dc circuit to the shop OCS.
B.
Stinger Supply System
Fig. 12 and Fig. 13 show one line diagram and control schematic respectively for the shop dc power
supply to shop stinger system. Fig. 15 show wall mounted dc disconnect switch ANSI device (89)
which is electrically interlocked with a companion dc disconnect switch (89) located inside the dc
switchboard by use of a 125V dc contactor. The control schematic of 125V dc contactor ANSI device
(29) enclosure with additional interlocks is shown in Fig. 16 and is part of local controller for
energizing and de-energizing the stinger’s 750V dc power receptacle. Interposing control relays and
indication lights can be used so that contactor operation is safe from switching in the dc contactor
contact in a no load condition for establishing the 750V dc circuit to the vehicle power receptacle.
C. General Discussion of Vehicle 750V Power Supply
Author is presenting the dc contactor control schematics to complete the 750V or 1000Vdc current
path with help of a dc contactor installed inside a local wall mounted enclosure to avoid hazard of
completing the circuit by operating 750 V or 1000Vdc disconnect switches. Design engineers can
have slightly different control schematic to energize and de-energize the contactor, however, each
designer is using 120V ac control power and then using suitable rectifier to provide 125V dc control
power for the contactor. Number of interlocks in the control circuit can vary based upon the
physical layout of the shop equipment. For safety enhancement; additional interlocks with
maintenance shop fall gate switches and monorail crane switches could be implemented.
Adequate electrical interlocks appears to be justified even if control interface wiring become more
involved with interlocks as safety of operators is improved against the malfunctioning of the fall
gates and malfunctioning of the monorail near dc energized components. These interlocks can be
described as follows:


Shop DC disconnect switches closed in the de-energized condition only.
Warning signs, beacon lights, and mechanical interlocks can be added not to open or close the
750V dc disconnect switch in the energized condition at the individual disconnect switches
and switches inside the dc panel board. DC disconnect switches inside the dc switchboard can
be interlocked with the dc switchgear feeder breakers.
16


Low voltage interlocks are provided with the door switch to the controller and maintenance
shop dc disconnect switch handle contacts.
Interlocks are also used with maintenance shop gate and the energized monorail crane to
assure shop gate is opened and closed safety from the hazard of 750V dc and monorail is not
operated when OCS is energized.
Each transit facility may have slight variations of the design of the contactor control and interlocks;
however, ultimate objective of the design is identical. Based upon author’s review of three
installations, it appears that the industry experts and maintenance operators should gather their
collective thoughts to make uniform standardized recommended design guide that is safe, reliable,
cost effective and easy to operate and maintain. Future research and development may minimize
human exposure to hazards of 1000V dc in the maintenance shop.
Light rail projects operating at 600V dc to 750V dc with OCS system can have vehicle equipped with
or without single pole 1000V dc rated power receptacle with proper cap and interlocks. When
vehicle is in the maintenance shop then dc power supply to the vehicle can be provided by using
750V dc matching single pole female plug to those vehicles which are equipped with the proper
matching power receptacle. Those vehicles that do not have power receptacle will receive power
from the OCS section located inside the maintenance building directly above the maintenance bay
and the track rails. In each case 750V power supply to the vehicle inside the maintenance building
is provided from a maintenance shop rectifier substation. DC feeder breakers inside the dc
switchgear located inside the rectifier substation are used to provide power to dc switchboards
rated at 750V dc. These dc switchboards are equipped with several individual disconnect switches
and individual fuses. Interlocks should be added that the fuses can’t be pulled out unless the
upstream switching device opens and makes de-energized circuit. Such a device is not depicted in
the figures included in this paper.
In addition, there is no assurance that 1000V V dc fuses shown in the shop dc switchboard will
provide proper protection for the low level arcing fault. Therefore, there is need for reviews that
the 1000V dc fuse application as shown is proper. These fuses can impose arc hazard if not properly
fitted in their house or maintenance person by mistake pulls such a fuse under energized condition.
17
Fig. 11 Maintenance shop 750V DC - OCS power supply one line diagram
18
Fig. 12 Maintenance shop 750V DC – Stinger system power supply one line diagram
Fig. 13 Control schematic of a dc contactor for the shop OCS or stinger dc power supply
19
Fig. 14 Layout of the wall mounted dc disconnect switch enclosure used for the maintenance
shop dc power supply
Fig. 15 Power supply to vehicle by using wall mounted enclosure with plug for the stinger system
20
Fig. 16 Control schematic of dc contactor for the maintenance shop dc power supply to stinger
system with additional safety interlocks with monorail crane and gates
VI
MAINTENANCE FACILITY - DC POWER SUPPLY SYSTEM 3rd RAIL SYSTEM
In case of heavy rail system, dc power supply to the train inside the maintenance building is
normally provided by using dc plug and receptacle assembly to keep 3rd rail outside the shop
building.
21
Local controller with a selector switch to open or close the dc supply circuit by energizing or deenergizing the dc contactor which in turn closes or opens the dc contacts wired in-line with dc
supply to the power receptacles. Adequate electrical interlocks are implemented for safe dc power
supply design.
Heavy rail (3rd rail) maintenance shop stinger system is described in this section. One track with
four individual floor pits under the track rails, and five emergency push buttons are shown in the
floor plan, see Fig. 17. One 150 KW 1000V dc power supply stinger unit consisting of 480V power
supply breaker CB1, contactor K1, manual primary voltage tap selector, rectifier transformer
(isolation transformer), 6-pulse rectifier unit, 1000V dc output terminals and metering units along
120V ac power supply control system including DC contactor for opening and closing the 1000V dc
power supply are all installed in a floor mounted enclosure (enclosure not shown), see one line
diagram of the 1000V dc rectifier in Fig. 21 and control schematic of the same rectifier in Fig. 22.
The floor mounted rectifier unit then provides 1000V dc power supply to 150 kW dual stinger
power supply unit, see figure 19 showing 150 kW dual stingers power supply and service stinger
stanchions. See Fig. 19 for the stinger cable assembly schematic diagram for connecting PSU to
stanchions (Fig. 18) and then to vehicle where negative return feeder from the vehicle to PSU is
indicted by others.
Fig. 17 General layout of one maintenance track showing location of emergency pushbuttons,
location of power supply unit(PSU),stinger stanchion cabinets, and stinger cables
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Fig. 18 Stanchion cabinet for use of stinger cable connection
Fig. 19 Simplified control of dual stinger power supply unit cabinet (PSU) with local control
Nomenclature:
A
Transit vehicle positive feeder connector (female power receptacle)
B
1/C - 2 kV unshielded copper wire #2AWG (positive feed)
C
Watertight male plug, 1000V dc, 100A, 2-pole & ground plus pilot pins
Fig. 20 Stinger cable assembly schematic diagram
23
21. One line diagram and control of 480V ac to 1000V dc power supply system
24
Fig. 22 Control schematic of the rectifier and 1000V dc power supply unit
25
D.
ARC FLASH HAZRAD ANALYSIS
Arc Flash Hazard Analysis is related to electrical safety improvement by design and then to use
personnel protective equipment (PPE) during maintenance to enhance protection of the
maintenance person from the electrical arc hazard that may take place during maintenance of the
electrical equipment. There are recommended standards for performing ac arc flash hazard
analysis and continuous work is going on all round the world through IEEE on electrical safety to
help the industry to improve personnel safety and to protect equipment [10] . However, dc arc
hazard analysis standards have not been developed so far for the rail industry [10].
There is a great need for such standards as dc power supply is used in the rail transit system, in the
ship on board distribution systems and in many other chemical plants and industrial systems.
VI. CONCLUSIONS AND RECOMMENDATIONS
Following conclusions and/or recommendations are provided for:
1)
2)
3)
4)
5)
6)
7)
8)
9)
There should be an AC HEP power supply to the diesel locomotive from a HRG grounded
power source when the vehicle is in the maintenance shop
There is a need for the APTA recommended guidelines for dc power supply to the vehicle in
the maintenance shop.
There will be minimum damage due to ground fault and maximum safety to the personnel if
HEP power is supplied from HRG grounded power system.
Underground 4/0 grounding connections between the HEP power supply equipment
enclosures bonding with track running rails should not be installed. Otherwise HRG resistor
will be bypassed.
The grounding conductor shown between main switchboards to shore power pedestal is to
meet NEC code requirements and will not affect HRG system so long installation of ground
conductor and the transformer neutral use HRG resistor before it get connected to the
transformer enclosure as shown in Fig. 6.
It is recommended that the existing APTA standards [2] [3] should make it clear that HEP
power supply should be from a HRG grounded power source. Use of an isolation transformer
shown in Fig. 3 can be added to these standards for the benefit of the design engineer in the
transit industry.
Ungrounded power supply system as described in some of the APTA documents [3] [4] [5] is
not a proper choice as it can cause insulation failure with transient overvoltage far more the
capability of vehicle wiring system. This should be reviewed and corrected.
For dc power supply to shop, local dc contactor controller with proper interlocks for safety of
the operator and protection of supply equipment with visual indication at the controller and
inside the shop are required.
Maintenance operator inside the shop should be trained for dc arc hazard analysis and use of
PPE while handling operation of dc controller.
26
10) Rail transit industry should get involved in developing APTA/IEEE standard for dc arc hazard
analysis similar to IEEE Standard 1584. This will help the industry for proper dc arc hazard
analysis and mitigate safety problems we encounter today from dc arc hazard.
11) APTA recommended guidelines for dc power supply to maintenance shop is needed for
uniform design and proper equipment enclosures with UL labels. Without such recommended
guidelines equipment may prove hazardous and design may prove inadequate. Author came
across non-uniform design in different transit agencies maintenance shop due to lack of
recommended guidelines.
ACKNOWLEDGMENT
Author would like to thank Hailu Keremo of AECOM in developing many figures used in the paper.
Many thanks to Kinh Pham of Elcon Associates, Manjit Khalsa and Krishan Sabherwal of VTA for LRT
shop OCS and stinger system design which author was involved in the design many years ago and
finally Vitaly Lusherovich of BART for discussing 3rd rail dc power supply to shop stinger system as it
was designed by Kaiser Engineers which is now AECOM.
REFERENCES
[1]
IEEE Std. 142, “Recommended Practice for Grounding of Industrial and Commercial Power
Systems”.
[2]
IEEE Std. 1584, “IEEE Guide for Performing Arc Flash Calculations”.
[3].
APTA-PR-E-RP-015-99 Edited 3-22-04, “Recommended Practice for Head End Power Source
Characteristics”.
[4]
APTA-PR-E-RP-016-99 Edited 3-22-04, “Recommended Practice for 480 VAC Head End Power
System”.
[5]
APTA-PR-E-RP-018-99 Edited 3-22-04, “Recommended Practice for 480 VAC Head End Power
Jumper and Receptacle Hardware”.
[6].
Dev Paul, "Operational Safety and Maintenance Considerations for People Movers, D-C
Grounding Systems", American Public Transit Association's (APTA) Rapid Transit Conference,
Buffalo, New York, June 1988
[7]
Dev Paul, "DC Traction Power System Grounding", IEEE Transactions on Industry
Applications, Vol. 38, No. 3, May/June 2002, pp.818-824
[8]
Dev Paul, "DC Rapid Transit System OCS Pole Grounding Technical Analysis and Safety",
American Public Transit Association's (APTA) Rapid Transit Conference, San Jose,
California, June 2003
[9]
Dev Paul, “How Effective is Automatic Grounding Devices at the Floating Negative Rail
System of a DC Powered Rail Transit System?” APTA Conference Vancouver B.C June, 2010.
27
[10]
Dev Paul, Peter Sutherland and Sergio Panetta, “A Novel Method of Measuring Inherent
Power System Charging Current” IEEE Transactions on Industry Appl. Vol.47, No. 6, Nov.
/Dec. 2011, pp 2330-2342.
[11]
Dev Paul, Electric Arc and Associated Hazards in the Rail Transit. Are we up to date with
current developments? Paper presentation at the APTA Conference, June 2011, Boston
[12]
Dev Paul, Electrical Design of a Rail Transit System based upon the criteria, “Design by
Safety”. Paper presentation at the APTA Conference, June 2012, Dallas Ford worth, Texas.
[13]
Dev Paul, “High Resistance Grounded Power System Equivalent Circuit, Damage at the LineGround Fault Location (Part 1)”, IEEE Transactions on Industry Appl. Vol.50, No. 6, Nov.
/Dec. 2014, pp 4179-4187
[14]
Dev Paul and Peter E Sutherland, “High Resistance Grounded Power System Equivalent
Circuit, Damage at the Line-Ground Fault Location (Part 2)”, IEEE Transactions on Industry
Appl. Vol.50, No. 6, Nov. /Dec. 2014, pp 4188-4196.
[15]. Dev Paul and Ben Chavdarian, “Ground Fault Current Protection of a 480Y/277V Power
System, Compliance with NEC Design Requirements”. This paper was first presented at 2013
PCIC conference, later it was presented at I&CPS 2014 conference. Now the same paper
titled, “Under current Protection Power System has been published in the IEEE Industry
Applications Magazine, Vol. 21, and No.1 ISSN 1077-2618, Jan. /Feb. 2015. pp. 23-32. This
year the same paper got selected for best paper power system protection paper award at
I&CPS 2015 May Conference in Calgary, Canada.
[16]
Dev Paul “High-Resistance Grounded Power System” Paper approved for publication in the
IEEE Transactions on Industry Appl. Vol.xx, No. x, Nov. /Dec. 2015, pp xxxx. This paper has
been requested by IEEE for special issue on Power System Grounding Papers
28