Surge Current Characteristics
570 Hybrid SPD
White Paper – April 2013
Abstract. This document will analyze the
surge current mitigation characteristics of the 570
surge protective device (SPD).
Included is a
description of the clamping and surge current
characteristics of the metal-oxide varistor (MOV) and
the silicon avalanche diode (SAD) surge components,
as well as, the design philosophy behind the unique
combination of the different surge technologies.
Introduction. It is well known that the electronic
devices utilized to enhance and protect our
standard of living are vulnerable to electrical
distribution fluctuations and anomalies. These
anomalies,
like
lightning
induced
and
magnetically coupled power line transients, are a
source of electrical equipment degradation and/or
malfunction.
Transients on the electrical
distribution network have many characteristics
and have been identified by the Institute of
Electrical and Electronic Engineers (IEEE) in [1].
Four waveforms associated with lightning
induced transients have been identified by the
IEEE [1]. The first and second transient waveforms
are described by the combination wave,
consisting of a specifically identified open circuit
voltage and short circuit current, represents
transients, which are most likely to occur at the
facility’s service entrance and on the electrical
distribution network at positions less than twenty
meters from the service entrance, see Figure 1
and Figure 2. The third transient waveform, the
ring wave, represents transients, which are most
likely to occur on a facility’s electrical distribution
network at distances greater than twenty meters
from the service entrance. The fourth transient
waveform is the long wave transient. This is an
additional transient that may also occur at the
service entrance of a facility.
National [2] and international [3], [4]
performance and safety standards used to design,
develop, test, and specify surge protective devices
(SPDs), utilize the lightning induced combination
wave transient. The combination transient has
unique open circuit voltage and short circuit
current characteristics. The open circuit voltage
has a rise time of 1.2 s and a time to decay to fifty
percent of the peak amplitude of 50 s. The short
circuit current has a rise time of 8 s and a time to
decay to fifty percent of the peak amplitude of 20
s. The open circuit voltage and short circuit
current amplitude ranges vary from specification
to specification. Common amplitudes identified
by the IEEE at various locations within a facility are
shown in Table 1.
To mitigate the identified lightning
induced transients, all with different amplitudes
and energy delivery capabilities, a discussion of
the commonly utilized surge suppression
components is essential.
Surge Suppression Topologies.
A
variety of topologies and technologies have
historically been utilized to provide transient
protection. These topologies use devices such as
gas discharge tubes (GDTs), metal-oxide varistors
(MOVs), silicon avalanche diodes (SADs), and even
selenium rectifiers to divert lightning induced
transients. Each of these surge suppression
components has numerous characteristics that
are beneficial in particular applications, but not in
others.
To limit the effects of adverse
characteristics, some manufacturers have
combined two or more different surge
suppression technologies. Well-known examples
of these combinations are MOVs and SADs, MOVs
and selenium rectifiers, MOVs and GDTs. Other
combinations of the suppression components,
such as SADs and GDTs, have been combined, but
are not as common.
Although the combination of suppression
components is prevalent within the transient
suppression industry, many manufacturers have
not utilized all the potential benefits of these
combinations.
As an example, many
manufacturers utilize MOVs and SADs within their
SPDs. The typical topology uses SADs as the
primary means of suppression. The MOVs are
utilized as a backup protection. In the advent of a
large transient being induced on the electrical
distribution network, the SADs take the full
transient. If the energy capability of the transient
is greater than the energy handling capability of
the SADs, the SADs fail and are off-line. In this
scenario, there has been a failure of the SPD, but
the equipment or facility is still protected by the
MOVs, which have been provided as back-up
protection. Traditionally, manufacturers have
designed their SPD systems such that
replacement of the SAD suppression components
can be accomplished relatively easily, provided
the required replacement items are available.
Although the concept of back-up
protection against transients with extremely large
energy delivery is very desirable, the failure of any
part of the SPD system is not. The replacement of
the SAD suppression components may be easy
and quick, but spare parts are required.
Furthermore, until maintenance is performed the
sensitive equipment deemed important enough
to have a SPD system requiring hybrid technology
is not fully protected.
The 570 hybrid SPD utilizes the full
capabilities of combining two different
suppression components: SADs and MOVs. To
fully understand the workings of the 570,
knowledge of the surge suppression components
characteristics are required. A description of each
device will be presented along with Emerson
Network Power’s patent pending technique
designed to utilize the beneficial characteristics of
each component.
Metal-Oxide Varistors (MOVs).
The
MOV has long been utilized to provide brute force
attenuation of lightning induced transients on the
AC power grid. The operational characteristics of
the MOV can be varied for application on different
voltages and transient exposure requirements. By
varying the thickness of the component, different
operating voltages can be obtained. Additionally,
by varying the diameter of the component,
different surge current capabilities can be
obtained.
Descriptively, the MOV is a bipolar,
nonlinear voltage controlled device. Activation of
the MOV occurs as the applied voltage increases
beyond the threshold voltage. Once the threshold
voltage is exceeded the MOV changes from a
high-impedance state to a low-impedance state.
The MOV continues in the low-impedance state,
diverting current between opposite polarity, until
the applied voltage has decreased to a value
below the threshold voltage.
The MOV is constructed from numerous
boundaries of specifically doped ceramic material.
These boundaries create breakdown regions,
which can be compared to those of a
“back-to-back” zener diode. By manipulating the
overall thickness of the device, and hence, the
number of boundaries in series, various
breakdown voltages can be obtained. Additional
manipulation of the overall width of the material
results in numerous parallel paths. The number of
parallel paths determines the total surge current
capability of the component.
To
determine
the
clamping
characteristics of the MOV [5], EQ1 is utilized
1
 I 
V  
k 
EQ1
where V is the clamping voltage of the MOV, I is
the current diverted through the MOV, k is an
intrinsic characteristic of the MOV, and  is the
slope which corresponds to the linear surge
current region of the MOV.
For MOVs,  ranges in values from nine to
twelve, with a typical value of ten. The value of k
ranges from 0.1x10-24 to 10x10-24. A typical linear
clamping characteristic for a 130 volt MOV is
shown in Figure 3.
Specific operational characteristics of the
MOV, in relation to other suppression devices, are
low-leakage current, no follow-on current,
moderate
clamping
characteristics,
high
-capacitance, high surge current capability, and
low-cost. The response time of the MOV can be
described as moderately fast.
Silicon Avalanche Diodes (SADs). The
SAD has been utilized for decades to provide
transient protection to lower-voltage systems,
typically located on a printed wiring board (PWB)
assembly.
The SAD can be configured for
numerous
voltage
and
surge
current
characteristics by adjusting the overall size of the
semiconductor p-n junction.
The SAD, like the MOV, is also a voltage
controlled device. Current is diverted through the
device once the threshold voltage has been
exceeded.
Once the applied voltage has
decreased to a level below the threshold voltage,
the device returns to a high-impedance state.
or high-current regions, the exact clamping
voltages of each device can be determined from
EQ1 and Figure 3. Once the clamping voltages
and the total surge current capability of the SPD
has been determined, an effective impedance can
be added to force the transition of surge current
from the low surge current capability of the SADs
to the high surge current capability of the MOVs.
This added impedance between the SAD and MOV
suppression components is the key to Emerson
Network Power’s 570.
A schematic
representation of the circuit is shown in Figure 4.
Mathematically, the impedance that
must be added in series to the SAD components
to force transient current sharing is determined by
VMOV  L
di
 Ri  VSAD
dt
EQ2
The SAD is constructed of a
semi-conductor p-n junction designed to operate
in the reverse breakdown region. In its purest
form, the SAD p-n junction is a unipolar device
with a specific forward and reverse voltage drop.
By applying two SAD p-n junctions in a
“back-to-back” configuration, a bipolar device is
constructed.
where VMOV is the voltage developed across the
MOV and the SAD device with the added
impedance, L is the added inductance, di/dt is the
rate of change of the applied transient current, R is
the added resistance, i is the transient current,
and VSAD is the clamping voltage of the SAD as
described in EQ1.
The clamping characteristics of the SAD
can also be characterized by equation EQ1.
However, the values of  and k are different.
Values of  range from 30 to 40, with a typical
value of 37. The value of k ranges from 1x10-89 to
10x10-89. A typical clamping characteristic for a
130 volt, bipolar SAD is shown in Figure 3.
Testing of the 570. To demonstrate the
surge current transition and survivability of the
SAD surge suppression components of the
Emerson Network Power 570, testing was
performed on a model 570YA16ARCG1S. Testing
was accomplished using transient generators
manufactured by Ketek Instruments and Emerson
Network Power Surge Protection.
Specific operational characteristics of the
SAD, in relation to other suppression devices, are
low-leakage, no follow-on current, tight clamping
characteristics, low-capacitance, low surge
current capability, and high-cost. The theoretical
response time of the SAD is less than most other
suppression devices, most notably the MOV.
The 570 Topology. As Figure 3 shows,
the clamping characteristics of the MOV and SAD
suppression
components
are
different.
Additionally, the MOV has a higher surge current
capability in relationship to an equivalent voltage
SAD.
When the suppression components are
operated in their linear region and not at the low-
The Keytek Instruments transient
generators were 587 Plus models with the S1/S3
and S7 surge module. Both generators developed
the IEEE C62.41-1991 combination wave
transient. The S1/S3 module generates an open
circuit voltage of 6,000 volts and a short circuit
current of 3,000 amperes. The S7 module
generates an open circuit voltage of 6,000 volts
and a short circuit current of 10,000 amperes.
The Emerson Network Power surge
generator also developed the IEEE C62.41-1991
combination wave transient.
This transient
generator develops an open circuit voltage of up
to 50,000 volts and a short circuit current of up to
50,000 amperes.
The various transient generators were
applied to the 570YA16ARCG1S SPD in the
line-to-neutral configuration. Transient current
was applied in increments up to a total of 29,000
amperes of 8/20 s current. Transient current was
measured and recorded at the input to the 570,
the input to the MOV module and the input to the
SAD module.
All data was recorded and is shown in
Table 2 and a graphical representation is shown in
Figure 5. As can be seen, the surge current has
been transitioned from the low surge current
capable SAD components to the high surge
current MOV components.
Conclusion. Combining different types
of suppression technologies, such as metal-oxide
varistors and silicon avalanche diodes, can only be
accomplished by thoroughly understanding the
inherent characteristics of the particular devices.
Different suppression technologies cannot just be
applied together in the same circuit and expected
to realize a combined benefit. In the case of the
570 SPD, a series impedance was added to
provide low clamping voltages and high surge
current capability. The added impedance allows
the transition of surge current from the primary,
low-clamping voltage, low surge current
capability
SAD
components
to
the
higher-clamping voltage, high surge current
capability MOVs.
Data shown in Table 2 and Figure 5,
demonstrates that the design principles utilized in
the 570 SPD truly provide transient transition
from the low surge current capability SAD
components to the high surge current capability
MOVs. This is contrary to the current industry
standard of providing primary SAD protection,
which must be replaced when subjected to high
surge currents.
For many years the 500 Series product
lines have been the premier SPDs in the market
place. They have provided low clamping voltages,
high surge current capabilities, and unsurpassed
quality and reliability. These traits have continued
with the development of the 570 Hybrid SPD.
References:
1. Institute of Electrical and Electronic Engineers, IEEE Recommended Practice on Surge Voltages in
Low-Voltage AC Power Circuits, Standard C62.41, dated 1991 February.
2. Underwriters Laboratories, Incorporated, Standard for Safety, Transient Voltage Surge Suppressors,
Standard 1449, second edition, dated 1996 August.
3. International Electrotechnical Commission, International Standard, Electromagnetic Compatibility (EMC),
Part 4: Testing and Measurement Techniques, Section 5: Surge Immunity Test , Standard 1000-4-5, dated
1995 February
4. International Electrotechnical Commission, International Standard, Surge Protective Devices Connected to
the Low Voltage Power Distribution Systems, Part 1: Performance Requirements and Testing Methods ,
Standard 61643-1, dated 1998 February.
5. Harris Semiconductor, Transient Voltage Suppression Devices, dated 1995 July
Tables:
Category
Open Circuit
Short Circuit
Pseudo Impedance
Voltage
Current
()
(kVpk)
(kApk)
C3
20
10
2
C2
10
5
2
C1
6
3
2
B3
6
3
2
B2
4
2
2
B1
2
1
2
Table 1.
Lightning Induced Combination Wave Transient Amplitudes and Effective Impedance.
Total Input
Current
(amperes)
342
359
2,620
4,340
6,060
7,788
9,540
10,944
11,944
12,840
13,828
14,836
15,824
16,832
17,808
18,896
19,496
20,440
21,448
22,432
23,280
24,408
24,648
25,768
26,776
27,600
28,928
SAD Module
Current
(amperes)
342
350
1,162
1,534
1,890
2,256
2,607
2,890
3,089
3,265
3,452
3,660
3,848
4,055
4,231
4,439
4,639
4,783
4,991
5,174
5,383
5,550
5,630
5,951
6,158
6,182
6,390
MOV Module
Current
(amperes)
0
9
1,500
2,840
4,200
5,560
6,960
8,080
8,880
9,600
10,400
11,200
12,000
12,800
13,600
14,480
14,880
15,680
16,480
17,280
17,920
18,880
19,040
19,840
20,660
21,440
22,560
SAD Module
Current
(percentage)
100.0
97.5
44.4
35.3
31.2
29.0
27.3
26.4
25.9
25.4
25.0
24.7
24.3
24.1
23.8
23.5
23.8
23.4
23.3
23.1
23.1
22.7
22.8
23.1
23.0
22.4
22.1
MOV Module
Current
(percentage)
0.0
2.5
55.6
64.7
68.8
71.0
72.7
73.6
74.1
74.6
75.0
75.3
75.7
75.9
76.2
76.5
76.2
76.6
76.7
76.9
76.9
77.3
77.2
76.9
77.0
77.6
77.9
Table 2.
Surge Current Distribution Between MOV Modules and SAD Modules of the 570YA16ARCG1S.
Figures:
C om bination W ave 1.2/50 O pen C ircuit V oltage
6000
Open Circuit Voltage (Volts)
5000
4000
3000
2000
1000
0
0
0.00005
0.0001
0.00015
0.0002
0.00025
0.0003
T im e (seconds)
Figure 1.
IEEE C62.41-1991 Combination Wave, 1.2/50 s Open Circuit Voltage at 6000 volts peak.
Short Circuit Current (Amperes)
C om bination W ave 8/20 S hort C ircuit C urrent
3500
3000
2500
2000
1500
1000
500
0
0
0.00001
0.00002
0.00003
0.00004
0.00005
Tim e (seco n d s)
Figure 2.
IEEE C62.41-1991 Combination Wave, 8/20 s Short Circuit Current at 3000 Amperes Peak.
MOV & SAD Clamping Characteristics
Clamping Voltage (Volts)
600.0
500.0
MOV
400.0
300.0
SAD
200.0
100.0
0.0
0
2500
5000
7500
10000
Surge Current (Amperes)
Figure 3.
Clamping Characteristics of Metal-Oxide Varistors (MOV) and Silicon Avalanche Diodes (SADs).
VMOV
R1
L1
3
3
1
4
VSAD
4
4
3
R
VR2
INPUT
VR1
2
5
VR3
5
5
5
Figure 4.
Schematic Representation of the Combination of the MOV and the SAD Surge Components in the 570 SPD.
Percentage of Surge
Current (Percent)
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
MOV MODULE
SAD MODULE
0
5000
10000
15000
20000
25000
30000
Surge Current (Amperes)
Figure 5.
Graphical Representation of the Transient Current Diverted Through the MOV and SAD Modules.
100 Emerson Parkway
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P (607) 721 8840
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