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ShaneR
Jan 28 2020 21:43

Hi all

Can somebody point me in the correct direction for selection of cables for "D" Curve MCB?

For some reason I though that you might have to go down one cable size when using "D" curve MCB?

Thought I might try for some facts on the matter.

Thanks all
   

DougP
Jan 28 2020 23:07

I'm not sure if this will appear as a reply to the post, or a new post - when I clicked on "add a reply", it seems to come up with the new post page instead of the reply page.

Anyway, a couple of relevant parts would be 8.3.9 loop impedance, and appendix B sections B4 and B5
   

DougP
Jan 28 2020 23:14

And the cable size may need to be larger for the same length of run, rather than smaller. Or possibly just have a larger PEC, to meet the lower Re required to match the breaker trip curve for automatic disconnection.
   

DougP
Jan 28 2020 23:15

So there is a glitch in the system.. the topic has disappeared.

It was: "D" Curve MCB and cable selection
   

AlecK
Jan 29 2020 09:33

There's no difference. The selection of overcurrent is for sustained overcurrent, so you use exactly the same rating of device; which must be no greater than the CCC for the cable in the prevailing installation conditions.

The "curve" of the device doesn't change the overload performance (thermal operation of the device) at all; it only affects the short-duration performance which is governed by magnetic rather than thermal trip.

C-curve magnetic performance allows for between 5.5 & 8.8 x the nominal rating. A D-curve will allow more current - between 10 & 14 x - to flow during start-up; which caters for in-rush or starting currents.

If that's not enough, you can move to magnetic-only operation, and provide overload by other means. I suspect that doesn't happen often these days, due to widespread use if soft starters & VFDs.

In the other direction there's B-curve, which has a magnetic trip between 3 & 5 x nominal rating. Useful for generators; where the genset may otherwise not be capable of supplying enough current to reliably operate short circuit or fault protection.
   

ShaneR
Jan 29 2020 10:04

"There's no difference. The selection of overcurrent is for sustained overcurrent, so you use exactly the same rating of device; which must be no greater than the CCC for the cable in the prevailing installation conditions."


Thanks