Fusing applications in SPD/TVSS products designed 
for parallel (panel type) hardwired installation

Short circuit fusing

Regulatory requirements

There are currently no mandatory requirements for short circuit fusing in an SPD/TVSS device designed for permanently connected parallel installation. Neither the Underwriters Laboratory (UL) Standard 1449 2^nd Edition, current ANSI/IEEE Standards, the NEC (National Electric Code), NEMA or the IEC require short circuit fusing.

What is required

It is required that SPD/TVSS devices be installed down-line from the facility's main/principal disconnect breaker. It is also highly recommended that all protective devices be installed in conjunction with a local disconnect breaker rated between 20 and 60 amperes. This will provide local fault-current protection and a means to service the device without shutting off the ac power to the installation. SPDs can also be equipped with integrated disconnects, fused or not.

Incorporation of short circuit fusing (What does this mean?)

The incorporation of internal and/or external short circuit fusing by SPD/TVSS manufacturers is, and has become, a major marketing gimmick to enhance and increase the sales of their products. Short circuit fusing adds little or nothing in additional safety provided; the SPD/TVSS device is certified to nationally recognized safety standards, i.e.; UL Standard 1449 2^nd Edition.

What short circuit fusing routinely does is cause a significant de-rating of the surge current capacity of the SPD/TVSS device. As an example, devices rated to handle surge currents from 80kA - 400 plus kA are usually limited in their performance by the incorporation of the short circuit fusing devices to >20kA. To purchasers of such products this means that they are making expenditures for performance that cannot be achieved, (regardless of the manufacturers' claims) due to the limitations of the short circuit fusing employed.

Electrical performance

Since 90 plus percent of all SPD/TVSS devices utilize Metal Oxide Varistors (MOVs) as the prime design approach for surge suppression, there exists a major incompatibility between the suppression technology and that of the short circuit fuse. For example, the MOV operates or responds typically (per manufacturers' specifications) within 50nanoseconds or less while the response time of short circuit fuses (per manufacturers' specifications) is in the millisecond range. This equates to no matter what the event, fault current, major transient temporary overvoltage; the MOV will fail before the fuse can operate.

In cases where the fuse has been oversized in order to pass industry test waveforms, (i.e. 10kA/20kV 8x20ms) the fuses performance is not superior and even in some cases the fuses do not perform as well as, the recommended breaker. Using the recommended breaker does and will provide the necessary over-current protection.

Should the fuse blow during a transient event, when current is rapidly changing, the sudden interruption of the transient current creates a very large transient upstream. Due to the inductance of the wiring, this can be fatal to the protected equipment/site. Once the fuse opens the site and its valuable equipment is completely without protection.

If transient over-voltages are considered a real threat then choose a SPD/TVSS design to withstand the maximum overvoltage stress that the equipment can afford. However be sure to select a product that has been certified to applicable safety standards.

Commentary: Failure mechanisms of MOV suppression components

MOV suppression components may fail in one of the following three modes when subjected to conditions that exceed the capacity of the device:

Open circuit failure: An open circuit failure is when the surge component is physically blown off of the circuit board or damaged so that the result is an open circuiting of the device.

Short circuit failure: A short circuit failure of the suppression component is a failure resulting in the component or device failing so in that the event leaves a short across the component. During a short circuit, either a fuse or the circuit breaker immediately upstream will operate, isolating the component/device.

High impedance failure: A high impedance failure is any failure of a suppression component leaving the device in a high resistance (impedance) state. Typical values of resistance in this state are 10 ohms to 20K ohms. The ac line current will flow though the failed device, but this fault current is not of sufficient magnitude to operate the fuse or breaker. The consequence of a high impedance failure is that the temperature of the component is elevated due to the limited current flow through the device. Some type of temperature sensing device to remove power from the suppression component is required to neutralize this thermal runaway condition.