Short circuit bypasses mcb and trips submain mcb

[Questforknowledge]

Good evening

A cable was loose in a ceiling void causing a short circuit this didn't trip the 10 amp 6ka type b mcb but tripped the 50 amp type b 6ka mcb that supplied that board back at the mains.

I am wondering why this occurred other than me thinking the local protection isn't very sensitive or
Functioning efficiently.

Cheers for any responses

 

[DPG]

How would the 100A main switch trip?

 

[Questforknowledge]

Not sure why I wrote that I just meant it bypassed the mcb and went back to the submain

 

[Risteard]

BS EN 60898 circuit breakers in series won't provide Selectivity for fault currents, so it's not entirely surprising that the fault takes out the upstream 50A device.

 

[DPG]

Not sure why I wrote that I just meant it bypassed the mcb and went back to the submain

It was a short circuit though, not an over current fault

 

[R-fur]

Is the main switch an RCD or RCBO?

 

[Julie.]

Mcbs or rcbos will not discriminate from each other at high currents, once you get into the instantaneous section of the trip characteristic.

The let through from the downstream rcbo/mcb is almost always more than the pre-arcing i2t required for the upstream rcbo/mcb

The same is true for rcd/rcbo in terms of earth fault current

 

[davesparks]

Good evening

A cable was loose in a ceiling void causing a short circuit this didn't trip the 10 amp 6ka type b mcb but tripped the 50 amp type b 6ka mcb that supplied that board back at the mains.

I am wondering why this occurred other than me thinking the local protection isn't very sensitive or
Functioning efficiently.

Cheers for any responses

It's pot luck which one will go first when you put standard mcbs from 6A to 63A in series with each other.
This is why submains should be protected by either fuses or MCCBs which have been properly selected.

 

[static zap]

once you get into the instantaneous section of the trip characteristic.

This does make me wonder if another load on the sub-main
took up mechanical free-play in the magnetic part... giving it a time advantage !
(vs free play allowing more velocity in local one )

(- or are we heading into physics of coil size-)
( -Smaller in higher current==Less inductance to overcome peak Current)

 

[pc1966]

Julie has it spot-on.

If you need discrimination for MCB then the up-stream one has to have some additional delay (as for the type-S RCD) otherwise a high fault current can trip whichever is quickest (or both).

With a fuse & MCB you can sometimes find that for increasing fault currents you get:

1. Small overload, few seconds then fuse trips before higher rated MCB
2. Medium current, MCB instantaneous trip goes before the fuse
3. High overload, fuse blows before the few milliseconds for the MCB trip