5 KA short circuit amps / symetrical

[lefty51]

Received a call today from maintenance. They are going to be hooking up an convection heater. Rating is 55 FLA.
On the front of the panel, it states " short circuit amps - 5KA - symetrical ". Inspector mentioned something to one of the maintenance personnel, that we may have to install a transformer to reduce this?? The feed is fed from an MCC with HRC fuses - IR capacity of 200KA. Why would be need to install a transformer ? There is the main feed from the MCC ( fused ) , then a main switch at the unit itself.... and also fuses inside the console. Am I missing something here. If the unit is capable of putting out 5KA under short circuit conditions, but the fuses are capable of interuppting more than 5 KA, then there should not be a problem..correct ? Possibly the maintenance person got his wires crossed...????

 

[tem1234]

The heater don't have nothing to do with the short-circuit current, The 5kA only means that the pannel can resist "without" damage to a short-circuit current of 5kA or less.

So you must calculate the short-circuit current at the pannel and confirme that it is less than 5kA symetrical. If the current is more than 5kA, well maybe you will have to add a transformer to reduce the short-circuit current, or install a pannel with a bigger rating, or...

The firt step is to calculate the short circuit current.

 

[davidbeach]

No, it can only be applied on a circuit that can supply no more than 5kA into a fault in the unit. If it isn't listed for use with those fuses on a circuit with more than 5kA you need to get your fault current down below 5kA. Transformer is one method; lots of wire can be another way.

 

[MikiBg]

Ok, it is clear that if prospective (calculated) short circuit current in place of panel/unit connection is more than 5kA we need to reduce it somehow. But what if some of protective devices listed above have cutting current characteristic? Could that limiting value, given from the manufacturer of protective device, be taken as valid for installation? Another question is why transformer, why not a choke if short circuit current mitigation is only aim?

 

[dpc]

Because the impedance of the reactor is always there even when there is no fault. You can use a reactor, but must account for the losses and voltage drop during normal operation. It may take quite a bit of impedance to get the fault current below 5kA.

If this is new equipment, you might check with the manufacturer and inquire about tested combination using an upstream current-limiting fuse. They may have already done some testing.

Next time, put the available short circuit current in the purchase spec.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[lefty51]

I have contacted the manufacturer about any testing which may have been done. ( The only time I was involved in this, was when I was called over after the unit was delivered.) Im sure they will make a note of this in the future.
Yes, I see what is missing now....HRC fuses can safely interrupt 200KA....but we have to limit SCCR to 5 KA somehow.

 

[MikiBg]

You don't have to if you prove by calculation based on real (installed) parameters that max symetrical short circuit current for fault at the point of heater panel connection would not be higher than 5kA. That is technicaly correct, what will inspector say/demand is the other thing. Or u already know it's higher? You didn't say anything about prospective sccr so far, just max permissible.

 

[lefty51]

No, I do not have anything from the manufacturer, other than what is posted on the door as mentioned in the first post.

 

[MikiBg]

No offense, but you're missing the point. Manufacturer already took his responsibility by claiming: my equipment (panel) will withstand any symetrical sccr up to 5kA. He can't tell what ACTUAL sccr will be in ur installation (or any other) at point of panel connection, that is something that u should know/calculate/estimate/measure. If u prove it is less than 5kA, no action required, just connect the panel, if it is more than 5kA, u need to reduce it by transformer/choke/feeder length extension.

 

[lefty51]

No offense taken. Will see what we can do to calculate the sccr with the engineering staff we have. Thanks!