Adding a Fuse to Fix SCCR 😟😟😟

 

This is an AI-generated image of a fuse in a cape.
Summary: Have you ever realized after the fact that a higher SCCR is needed? In this blog we will discuss the use of fuses, and why this may not fix insufficient SCCR.

What's the problem?

The NEC requires that control panels have sufficient SCCR. This is the amount of short-circuit current a control panel is rated to endure should a short-circuit occur. There could be a lot of available short-circuit current or a little. It all depends on what the electrical system can deliver. Usually, the cause for the panic comes when it is discovered that a higher SCCR is needed AFTER the control panel is built and installed in the field. A simple quick fix would be ideal in this situation. Often the idea of adding a fuse comes up.

Why add a fuse?

A fuse in the simplest terms is a strip of metal, usually copper or some alloy, in a protective tube that melts very fast in the event of a steep rise in current, like a short circuit. This quickly opens the circuit and halts the damaging high current levels. Sometimes the fuse contains a sand material, which turns to glass in the presence of high heat (which occurs when the copper melts and arcs). Glass is highly resistive to current, so this helps extinguish the current even faster. So this should fix any SCCR issue right? Unfortunately, no.
Why this doesn’t this fix it?

Adding a fuse may mean the short circuit event is halted more quickly, but it doesn’t mean the other devices will survive without harm.
  • Overcurrent protective devices, such as a fuse or breaker, are designed to open during a short-circuit event (if properly applied, of course).
  • Other devices, like motor starters, power distribution blocks, disconnects, etc, are not designed for high current events. They could fail catastrophically that could lead to personnel injury, damage to other devices or fire.
  • A fuse upstream of the control panel may not help smaller sized devices.  Let's suppose a 600 amp fuse is added upstream of a control panel with a 10 kA miniature circuit breaker sized a 0.5 amps.  The miniature circuit breaker isn't going to wait for the 600 amp fuse to open the circuit before responding when the high short-circuit current is abated.
  • Spinning motors act as generators during a short circuit event.  A fuse upstream of the control panel may not be located in the circuit between the spinning motor and short-circuit location.
  • There are some exceptions that allow the addition of a fuse to raise the SCCR of some devices, but the conditions limit the usefulness of this approach. It won’t fix feeder items nor raise the interrupt rating of another circuit breaker.

🔧 So what is the fix?

Avoid the mistake in the first place… plan out your SCCR ahead of time. If you don't have that luxury (or a time machine), then a transformer may help solve the problem. Adding a transformer ahead of the control panel has the effect of lowering the available fault current. This can sometimes negate an insufficient SCCR.

You don’t have to go at this alone though. Send us a line if you would like training or want to discuss SCCR.

🔎 For more information check out the following: 


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