Bulletin EC
The 1993 National Electrical Code Mandates:
Elevator Circuits Must Be
Selectively Coordinated
200 A Sub-Feeder
100A
Branch
EL-1
EL-2
EL-3
400A
Feeder
Utility
Transformer
1600A
Main
“In most commercial and industrial facilities, a system blackout would be intolerable.”
“Selective Coordination is always more desirable than a system blackout.”
Selective Coordination
What is Selective Coordination?
Today, more than ever, one of the most important parts
of any installation - whether it is an office building, and industrial plant, a theater, a high-rise apartment or a hospital - is
the electrical distribution system. Nothing will stop all
activity, paralyze production, inconvenience and disconcert people and possibly cause a panic more effectively than a major power failure.
We may define selective coordination as “THE ACT OF
ISOLATING A FAULTED CIRCUIT FROM THE REMAINDER
OF THE ELECTRICAL SYSTEM, THEREBY ELIMINATING
UNNECESSARY POWER OUTAGES. THE FAULTED CIRCUIT IS ISOLATED BY THE SELECTIVE OPERATION OF
ONLY THAT OVERCURRENT PROTECTIVE DEVICE
CLOSEST TO THE OVERCURRENT CONDITION.”
Figure 1 illustrates a non-selective system (left) and a
selectively coordinated system (right).
Most designers and specifying engineers will agree
that a system blackout in most commercial and industrial facilities would be intolerable.
Selective Coordination and the NEC
Prior to the 1993 NEC®, several sections referenced
selective coordination. Section 240-12 (Electrical System
Coordination) and 517-17 (Ground Fault Protection in
Health Care facilities) are notable. These sections made
reference to requirements for selectively coordinated systems. However, total selective coordination was not mandated.
SELECTIVE COORDINATION
NON-SELECTIVE COORDINATION
(Total System Blackout)
Not Affected
A
Not Affected
A
De-energized
Portion of System
Also Opens
Not Affected
B
B
Also Opens
C
C
Not
Affected
Opens
Opens
D
D
Branch
Circuit
Fault
De-energized
Portion of System.
(This is the only part of
the system affected).
Figure 1
2
Fault
,
Elevator Circuits Must Be Selectively Coordinated
The 1993 NEC®, Section 620-15(a) states: circuits and
overcurrent devices which feed elevator controllers
shall be selectively coordinated. This wording is
unprecedented in the NEC in that absolute selective coordination is indeed required. A design engineer must specify
main, feeder, sub-feeder, and branch circuit protective
devices that are selectively coordinated for all values of
overloads and short circuits.
To understand this new mandate, we will examine the
system in Figure 2, and those circuits supplying elevator
controllers, specifically down to the 100 ampere branch
circuit.
200A Sub-Feeder
100A
Branch
EL-3
EL-2
EL-1
400A
Feeder
1600A
Main
Utility
Transformer
Figure 2
This Design Violates the National Electrical Code
Figure 3 consists of an Insulated Case Circuit Breaker
(ICCB) and Molded Case Circuit Breakers (MCCBs). Figure
4 shows a lack of coordination for any value of current in
excess of 750 amperes. Faults in excess of 16,000
amperes cause a total system blackout. Violations of section 620-51(a) exist.
400
300
200
100
80
60
1600A ICCB
40
400A MCCB
30
100A
MCCB
20
200A MCCB
100A MCCB
EL-3
400A
MCCB
2
1
.8
.6
.4
.3
.2
.1
.08
.06
.04
.03
.02
Figure 4
3
40,000
60,000
80,000
100,000
BLACKOUT
(PARTIAL)
CURRENT IN AMPERES
30,000
8,000
10,000
6,000
4,000
3,000
2,000
.01
600
800
1,000
Figure 3. VIOLATION
3
100
Utility
Transformer
1600A
ICCB
4
300
400
EL-2
200
EL-1
TIME IN SECONDS
10
8
6
20,000
200A Sub-Feeder
1,000
800
600
BLACKOUT
(TOTAL)
,
,
This Design Meets the National Electrical Code
Figure 5 consists of Class RK1 LOW-PEAK® fuses,
Studying Figure 6, it can be seen that selective coordination exists for these fuses. Compliance with Section 62051(a) exists.
1,000
800
600
400
300
200
LPS-RK400SP
LPS-RK200SP
100
80
60
LPS-RK100SP
LPS-RK200SP
KRP-C1600SP
40
30
.3
.2
.1
.08
.06
.04
.03
.02
300
400
200
100
.01
60,000
80,000
100,000
.4
30,000
40,000
1
.8
.6
20,000
2
KRP-C1600SP
8,000
10,000
Figure 5. COMPLIANCE
3
6,000
Utility
Transformer
4
3,000
4,000
LPS-RK400SP
10
8
6
2,000
EL-3
TIME IN SECONDS
EL-2
EL-1
600
800
1,000
LPS-RK100SP
20
CURRENT IN AMPERES
Figure 6
Summary
Selective coordination is always more desirable
than a system blackout. An analysis of overcurrent device
time-current characteristics is required to assure coordination. Selective coordination for elevator circuits is mandated
in the NEC Section 620-51(a)
The reader is directed to other sources for information
and assistance on performing selective coordination studies:
Buss Bulletin EPD-2, “Selective Coordination of Overcurrent Protective Devices for Low Voltage Systems” is a
manual on selective coordination.
Tron Computer Software for Power Systems Analysis
consists of a package of three PC based programs,
❚ One of these, BUSSPLOT®, performs Selective
Coordination Studies
❚ BUSSPLOT® analyzes fuse and circuit breaker time
current curves
❚ Outputs data to a printer or a plotter.
This Bulletin is intended to clearly present comprehensive product data and provide technical information that will help the end user with design applications. Bussmann reserves the right, without notice, to change
design or construction of any products and to discontinue or limit distribution of any products. Bussmann also reserves the right to change or update, without notice, any technical information contained in this
Bulletin. Once a product has been selected, it should be tested by the user in all possible applications.
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Copyrighted April 1993 by Cooper Industries, Inc., Bussmann Division. Printed in U.S.A.