THE IMPLICATIONS OF NEW TECHNIQUES 

Francois D. Martzloff 

Corporate Research and Development 

General Electric Company 

Schenectady, NY 12345 

Summary Report 664 [l] proposes voltage levels of 2500 V for a 

120 V circuit and 4000 V for a 220 V circuit (Table 1). 

Reliability problems can occur from the use of However, these voltages are not consistent with the 

modern electronic devices without applying appropriate inherent withstand characteristics of electronic appliprotection techniques or using incorrect installation pro- ances. A much lower level (indicated by Category I 

cedures. Although surge arresters are effective in limit- or I1 of the 664 report) is required, i.e., 800 or 1500 V 

ing overvoltages, a metal oxide varistor can provide a 

much lower clamping voltage if installation procedures 

are taken into consideration. Sparkover voltage measurements, with a specified time rise, measured arrester 

oerformance. The resDonse of the arresters to a 

current impulse was investigated and lead effects were 

identified. Tests indicated that the metal oxide varistor, 

installed with short leads, provides low clamping 

voltage. 

Introduction 

Incorrect protection for modern electronic devices 

from lightning strokes can cause reliability problems 

which could arise from various sources: 

Sensitivity of modern electronic equipment 

Improper procedures of installation 

Complete lack of protective devices. 

This paper examines new applications of old concepts Figure 1: Surge arrester for 120 V circuits, service 

which are required by the constantly increasing use of entrance or panel installation 

electronic equipment; the particular increased sensitivity 

of these devices; and intense, competitive pressures. 

We shall consider first the design and environment 

of surge arresters for low-voltage systems and then 

examine their performance as a function of installation. 

Surge arrester design for low voltage systems 

In the past, typical surge arresters (diverters) for 

service entrance duty have been limited to a gapvaristor design. This design involves gap sparkover 

voltage with a result of a relatively high clamping voltage for the arresters. The new, commercial availability 

of metal oxide varistors, with current ratings suitable 

for service entrance duty, provides a low clamping voltage at the service entrance. 

Surge arresters, which have sufficient current 

discharge capacity, consist of a gap in series with a nonlinear resistor, usually a silicon carbide block (Figures 1, 2, and 3). These arresters are effective in limiting overvoltages to levels compatible with solid insula- Figure 2: Surge arrester far 220 V circuits, panel 

tion. In recognition of this compatibility, the IEC installation 
Figure 3: Surge arrester for 220/440 V circuits, 

service entrance installation 

Gap 

Sparkover 

tme (us) 

for 120 V circu~ts and 1500 or 2500 V for 220 V circuits. These voltages can be achieved with a 32 mm 

metal oxide varistor for which the rated clamping voltage is 550 V and 900 V for disks suitable for 120 V and 

220 V circuits. respectively. 

However, the capability for low clamping voltage 

might not be attained if installation procedures do not 

take the connecting lead effects into consideration. 

Furthermore the proposed IEC practice of several casc ded surge protective devices requires careful coordiAtion of the devices and the intermediate 

impedance [21, a goal which may not be easy to achieve 

in routine installation ~ractices. 

Table 1 

Preferred series of values 

of impulse withstand voltages for rated voltages 

based on a controlled voltage situation 

Voltages L~ne-to-Earth 

Der~ved from Rated 

System Voltages Up to 

(V rrns and dc) 

Preferred Series of Impulse Withstand 

Voltages in Installation Categories 

Test procedures and standards 

The evaluation of surge arrester performance is 

accomplished by the application of standardized tests 

which are presumably specific to the operational 

environment of the arrester. 

Performance tests for a low-voltage arrester include 

sparkover voltage measurement with a specified rise 

time and also the use of one or more current impulses 

to demonstrate the capability of discharging a surge 

either without damage or without the production of 

excessive discharge voltage during the surge. Figure 4 

shows the relationship between these parameters of a 

gap-varistor design. Because damage to semiconductors 

is likely to occur during the initial front of the surge 

before sparkover, the concern over the following 

discharge voltage is less significant. 

Figure 5, however, shows how the gapless varistor 

can clamp at lower voltages. But, there is a risk of an 

inductive drop which would add a substantial voltage to 

Figure 4: Characteristics of conventional surge 

arresters 

n Surge current 

Varistor voltage 

time (ps) J 

Figure 5: Degradation of clamping voltage caused by 

misapplication 

the intrinsic clamping voltage due to the long connecling leads required under some proposed regulations [31. 

Sparkover voltage 

Figure 6 shows the sparkover voltage of typical 

arresters in USA circuits at 120 V line-to-ground and. 

also, in European circuits at 220 or 440 V between terminals. These sparkover voltages were recorded for a 

I (a) Open circuit 10kV I ~IS (b) Sperkover of 120 V Arrester 

at 1600 v m 550nS 

(c)Sparkover of 220 V arreste (d)Sparkover lof 440 V arrester 

at 1700 V in 480 ns I I at 1700 V in 420 ns I I I I 1 

Figure 6: Sparkover characteristics of conventional 

arresters 
10 kV/ps rate of rise (Figure 6a). It is apparent that 

the gap-type arrester oscillograms exhibit an anomaly at 

approximately 150 ps before the gap sparks over 

(Figure 6b, c, d).* 

In contrast, the clamping voltage of the varistor 

(Figures 7a and b) is not only lower, but it is also free 

from any interference. In Figure 7c, the absence of a 

significant overshoot in varistor clamping is shown: 

a The fast front is the open circuit voltage without 

the varistor 

a The trace to the right illustrates the clamping 

action of the varistor. 

Table 2 

Surge voltages and currents deemed to represent 

the indoor and outdoor environment and recommended 

for use in designing protective systems* 

Major feeders, 6 kV 

Load center 8/20 ps 1 3kA I 1.2150 ps 

Location 

Category 

Outdoor 10 kV 

installations 1.2/50 ps 

(a) Clamping voltage of 250 V varistor. 

Low-Impedance 

Circuits 

(b) Clamping voltage of 150 V varistor 

High-Impedance 

Circuits 

(c) Composite recording of the 

open-circuit voltage rise 

and response of the 150V 

varistor, expanded V scale 

Figure 7: Clamping voltage of metal oxide varistors 

Impulse current 

The selection of the current waveform is not obvious. The use of an 8/20 ps waveform to represent 

surge currents associated with lightning strokes is well 

established. Indeed, most standards [4,5] call for an 

8/20 ps waveform. Levels may be in the range of 3 to 

10 kA crest at the service entrance (Table 2) [4]. 

The selection of an 8/20 ps wave reflects our 

present day knowledge of typical lightning currents [6,71. In addition, the 8/20 ps wave discharges 

an appropriate amount of energy in the arrester under 

test. 

The question, then, of the likelihood of a 8 ps front 

propagating along a low voltage system can be raised. 

Figure 8 depicts a possible distribution of the surge 

current from a stroke to an overhead system. Taking 

50 kA [8] as the median level of lightning stroke, the 

resultant 5 kA crest is expected, and, with short distances along the service drop, a rise time of 8 ks can be 

maintained. 

* The explanation of this peculiarity is actually quite simple. In 

real time, the gap fires 150 ps before the display records the 

event, bur the oscilloscope used for these tests has a 150 ps 

delay line. Therefore, the anomaly is the interference created in 

the oscilloscope by the gap. (Even an EM1 option for the 

oscilloscope is not enough!) This occurrence exemplifies the 

object~onable effects that a gap can have upon electronic devices. 

-- I I 

'Reproduced In part from the IEEE Standard I41 

which calls for 3kA at the "load center" and 

10 kA at "outdoor installations." 

LEGEND 

LA PRIMARY ARREST 

XF DlST TRANSFORMER 

M METER 

Figure 8: Current division for a stroke to an overhead 

system 

Within these parameters, an 8/20 ps waveform for 

both the conventional arresters and the candidate metal 

oxide varistors in service entrance duty appears reasonable. In addition, it is likely to be demanded in the performance of test procedures for arresters - either by 

customers or by regulatory agencies. 

Installation of arresters in panels 

Two panels, typical of USA and European hardware 

(Figures 9 and lo), were wired in the laboratory and 

subjected to impinging surges of 5 kA crest, 8/20 ps 

(Figure 1 la), that were applied between one phase line 

and the panel ground. Voltages appearing at the outgoing branch circuits were recorded with oscilloscope 

probes by using a differential connection after preliminary checking on signal/noise performance of the system. Figure 1 lb shows the response of the 120 V 

arrester to this impinging surge. This response will be 

disscussed in detail with the test results. 
Figure 9: Typical 120/240 V service panel in USA 

practice, with arrester installed outside 

panel 

On the USA-type panel, the 120 V arrester was 

installed externally to the panel, and the 45 cm long 

leads were connected to the main entrance lugs of the 

panel (as implied by the specifications of the National 

Electrical Code and the proposed UL Document (31). 

The 220 V arrester is designed for installation in the 

panel, and the point-to-point wiring allows short leads 

for the connection across line and ground (or neutral) 

inside the panel. The 440 V arrester, as indicated by 

the manufacturer's suggested installation (Figure 12), is 

intended to be connected outside at the service 

entrance rather than at the panel. Consequently, in the 

laboratory simulation, it was connected 3 m before the 

panel 

Figure 12: Manufacturer's suggestion for 400 V 

arrester installation 

The 150 V and 250 V varistors (Figure 13) were 

installed either outside or inside the panel. The installation will be discussed with the test results. 

European practice, with integral arrester 

connection .. . . 

(a) 5kA, 8120~s current 

impulse applied for 

discharge voltage tests. 

(b) Discharge voltage of 

120 V arrester during 

8120~s impulse. 

Figure 11: Applied impulse and 120 V arrester 

response 

Figure 13: Metal oxide varistor (32 mm) packaged 

for industrial applications