Table 1-15. Test Results for Parallel-Connected ZNR
Clamp Voltage (Peak)
250A @ 1OOOV
400A @ 1600V
20, OOOA @ 8.75kV
40,OOOA @ 16.8kV
50, OOOA @ 20kV
Testing has established that connection of the devices listed in
(1) Current dissipation.
Table 1-13 in parallel for line-to-ground or line-to-line protection is feasible. Use of the ZNR in parallel
provides increased current dissipation capability and a lower maximum clamping voltage than a single, high-
energy ZNR can provide. Five of the devices were connected in parallel and surged as listed in Table 1-15. The
clamp voltages listed in Table 1-15 occurred. Current division was very good.
(2) Turn-on. Although the ZNR devices used in ZNR-type arresters are not solid-state junction-
type devices, the arrester acts very much like junction-type devices. That is, when breakdown voltage is
reached, transition from off to on occurs very rapidly as shown in Figure 1-39b which is a typical operating
curve for a ZNR. Since the devices used in ZNR-type surge arresters are essentially nonlinear resistors,
resistance decreases rapidly as applied voltage across the device increases above breakdown voltage.
Therefore, current flow through this type of arrester increases rapidly after breakdown as shown in
Figure 1-39b. Primarily because of resistance and capacitance of the ZNR, the clamp voltage slightly lags the
transient current waveform. The ZNR-type arrester automatically restores to the off state when applied
voltage falls below turn-on voltage. Therefore, no follow current occurs during the turn off phase.
So many different types of solid-state arresters are currently
Solid-state type arresters.
manufactured that it is difficult to generally evaluate them. In general, solid-state arresters manufactured by
connecting silicon avalanche diode suppressors (SAS) in series to attain the desired current handling capability
have truly fast response times of 1 to 10 nanoseconds. However, this type of arrester is generally limited to
handling approximately 500 amperes surge current (waveform 8-by-20 to 8-by-40 microseconds). Figure 1-39a
is a typical operating curve for a solid-state suppressor. This type of arrester also has a low clamp voltage
(normally 160% of breakdown voltage, maximum) compared to other types of arresters. Other solid-state
arresters are a combination of silicon avalanche diodes or rectifier diodes connected in a bridge network
followed by a second stage consisting primarily of a silicon-controlled rectifier (SCR) with a varyingvalue
current-limiting resistor in series with the SCR. This type arrester has a slow response time, sometimes
approaching 1 microsecond, because of the slow turn-on time for the SCR. Also, the clamping voltage can be