Testing of SPD Disconnectors

Testing of  SPD Disconnectors

Two of the more important international surge protection standards are:Underwriters Laboratories Incorporated, ANSI/UL1449-Surge Protection Devices, and IEC 61643-1-Surge protective devices connected to low-voltage power distribution systems(Part 1: Requirements and tests).

UL1449 provides three tests to evaluate an SPD’s ability to safely disconnect under simulated fault conditions.

UL Limited Current Test

This test is intended to simulate a specific high impedance fault condition which is unique to the North American power system.Many residential and light commercial installations in the US are fed with a 120/240V, 1Ph, 3W+G system derived from a centre-tapped transformer. This supply presents a particularly onerous set of problems to an SPD if it experiences the infamous “Loose Neutral” phenomenon-a problem which occurs if the neutral connection becomes corroded, or disconnected, and the loads connected between L1-N and L2-N are not balanced. Under such conditions,the zero point of the system shifts and the voltage on one“half” of the load will decrease, and that on the other “half”will increase. Under such conditions, the L-N voltage to which the SPD is connected may elevate above the nominal 120V and force the SPD into permanent conduction under a limiting current of several amperes.

The test also serves to simulates the ageing behaviour of varistors as their Uc characteristics change and they begin to conduct (clamp) on the peaks of the 50/60 Hz supply sinusoid.The test is performed by connecting the SPD to a current limiting supply set to 0.5, 2.5, 5 and 10A, with a “full phase voltage” (e.g. 240V for an SPD intended for use on a 120V3W+G system, or 480V for an SPD intended for use on a 277/480V 4W+G system). This voltage is applied for 7hours, or until the current to, or temperature of the SPD attains equilibrium, or until disconnection from the supply results. The SPD is required to pass safely-generally via the operation of the internal thermal disconnector.

UL Intermediate Current Test

A well designed SPD will generally include both thermal and over-current disconnectors - the former being to take the SPD off-line during limited current situations when the failure occurs more gradually over time, and the latter being to rapidly disconnected before failure of the internal active elements can cause excess short-circuit currents to flow and induce a potentially catastrophic explosion of fire hazard.Disconnection from currents of some hundreds of amps(intermediate currents) is generally difficult as both thermal and over-current disconnectors may be too slow to operate in this region. For AC-SPDs UL evaluates safe behaviour using: 100A, 500A and 1000A. The test protocol for DC SPDs is currently under consideration, but it is likely that devices will be tested at: 10A, ISCPV and 5 x Iscpv, where Iscpv is the prospective short circuit current of the photovoltaic panel it is rated for use with.

UL Short-Circuit Current Test

This test evaluates the ability of an SPD to disconnect itself from a power system which is able to supply large prospective fault currents. The aim is to ensure that disconnection occurs sufficiently fast to limit excess energy in the failed SPD, thereby avoiding an explosion or fire hazard due to follow-currents. The test involves instantaneously applying an elevated voltage to the SPD from a supply capable of delivering the full short-circuit current which the manufacturer wishes to have marked on his product. This simulates the race-condition which exists between the SPD’s over-current disconnector (fuse or circuit breaker) and the build up of explosive energy in the failed internal non-linear component (MOVs, SADs, gaps etc).

Similarly, IEC 61643-1 evaluates an SPD’s ability to safely disconnect using the following tests:

IEC Thermal Stability Test

This test is similar to UL’s limited current test in that it simulates the behaviour of the SPD when it reaches end-of life due to ageing of its internal non-linear components. The test involves progressively increasing the current through the SPD in discrete steps of 2mA, and allowing thermal equilibrium to be achieved at each point before moving to the next increment. Under such conduction, the SPD gradually increases its internal temperature to the point where either safe disconnection, or burning, occurs.

IEC Temporary Over-Voltage TOV Test

The TOV tests involve subjecting the SPD to various overvoltages which are intended to replicate those that can occur under various network faults. The SPD should either withstand, or safely disconnect, from these scenarios. The duration of time for which the TOV is applied is: 5s to simulate faults on the low voltage side of the distribution system and 200ms to simulate faults on the high voltage side(typical trip times of protection relays used on IEC regulated networks).

IEC Short-Circuit Current Test

IEC 61643-1 states that “an overstressed (short-circuited)SPD shall withstand the power short-circuit currents which may occur in service”. The testing entails sample preparation in which any voltage limiting components or voltage switching components are replaced by copper blocks (dummies). The modified sample is then connected to a power frequency source at the stated maximum operating voltage Uc and prospective short-circuit current Isc as declared by the manufacturer.The modified sample is energised twice (once at 45 andonce at 90 electrical degrees after the voltage zero crossing).If a replaceable internal or external disconnector operates itis replaced (or reset) and the test continued. Pass criteria is that there is no evidence of fire or burning.IEC Intermediate Current Test:To evaluate behaviour at low (intermediate) short-circuit currents, samples are again prepared with dummy copper blocks and energised at the maximum continuous operating voltage of the power system for five seconds4. The prospective short-circuit current is set to five times the rating of any up-stream over-current disconnector specified by the manufacturer (or 300 A if not specified). While this method has the right intention, detractors feel the method of replacing the active non-linear elements with shorting copper blocks limits its usefulness.