Diff:-Short Circuit Current 21 for 3sec and 21 for 1Sec

In context of the equipment rating, the time in sec is the time period the equipment can withstand the short circuit current. So, 21 kA for 3 sec is better than 21 kA for 1 sec. However, the short circuit current in the range of 21 kA are cleared less than 1 sec, so the 21 kA for 1 sec rating should even be fine. The heat generated by the short circuit is I² x R x t. The more time the short circuit current sustains, the more heat is generated within the conductor of the equipment because of its resistance, the more temperature raise occurs in the equipment. The temperature rise should be within the limit so that the equipment is not damaged. Generally the equipment has a damage curve which indicates the time in seconds the equipment can withstand for short circuit current. The relay setting is coordinated to make sure that the fault is cleared before the equipment is damaged. For the coordination, the relay curve is kept at the left side (or below) the equipment damage curve.

- MS

Sub: Short Circuit Current 21KA for 3Sec and 21KA for 1Sec.

Current flowing through a conductor produces heat in the conductor.

Example: Current flowing through transformer or motor produces heat.

So take square of Current multiplied by time and comapre in both cases (21 X 21 X 1 Amp ~ 21 X 21 X 3 ). The short circuit current for 3 seconds produces three times more heat than one Second and hences damages the conductor. So you need to provide a HRC (High rupturing Capacity) fuse or Circuit breaker to clear the fault with out damage to the equipment. Higher SC currents for longer times makes inter turn movement and thus due to rubbing of conductors, insulation of the conductor gets damaged.

This leads to insulation / equipment failure after two three times.

J.N.Karamchetti Hyderabad.

MSAMAD qoute " So, 21 kA for 3 sec is better than 21 kA for 1 sec."

I would like to know why you think this is so. 21ka for 1 second gives you arcing damage, a lot of black smoke, some conductor damage, worker probably ok.

21 ka for 3 seconds give you bad arcing damage, panel destroyed, smoke and fire, conductor major damage, workers had left town.

Thanks for the question. Here is the explanation:

This is what I said is 'in context of the equipment rating' (see the beginning of my post). If an equipment (say cable or busbar or breaker etc.) is rated '21 kA for 3 seconds', it wouldn't damage in the case of 21 kA flows through it for 3 seconds (so no smoke, no damage). However, if the equipment is rated at '21 kA for 1 second', it will damage if the same current flows for 3 seconds. The damage curve of the equipment of '21 kA for 3 seconds' stays above (or right) the damage curve of the equipment '21 kA for 1 seconds' and hence it provides more flexibility for device coordination.

AND, of course, the fault should be cleared as fast as possible (as it allows for the device coordination study so that the upstream protection doesn't operate earlier than the downstream protection). So, in the context of fault clearing, the less time the fault sustains, the better for the equipment of the faulty circuit.

Hope it clears.

- MS

Very clear. I misread your statement.

Hi all ,

Thanks for your post

In one of the my project ,I supplied a panel which having 21kA for 1Sec and the customer is asking me to provide 21kA for 3 sec instead of 1 sec.How can i sought the issue?