RCD/mcb

[vbyrne]

can anyone tell me ,what kind of a fault blows a RCD ,and what kind of fault blows a mcb

 

[Des 56]

Rcd, an imbalance of the live + neutral of the rcd
It could be a live-earth fault or a neutral-earth fault ( it takes little fault current to trip it out)

If you want extreme circumstances for the word "blow" to be approporiate
A dead short of the live and neutral of the outgoing of the rcd,

MCB Will operate when the circuit design current is exceeded for a long period or
When a short circuit( live-neutral) occurs to the circuit protected by the mcb
When a live- earth fault causes a whack of fault current to flow

 

[Deleted member 26818]

I've always described it as earth leakage for the RCD, and overcurrent for the MCB.

 

[vbyrne]

so Des ,a RCD that has operated is a voltage fault on one of the circuits its covering ,and a mcb that has operated is a over current fault

 

[vbyrne]

a fault from the phase or neutral to earth ,is that what will cause the rcd to operate

 

[Des 56]

so Des ,a RCD that has operated is a voltage fault on one of the circuits its covering ,and a mcb that has operated is a over current fault

As spinlondon answered "leakage" an rcd operates when there is leakage to earth from the balanced live-neutral,either live-earth or neutral-earth

T

 

[daiplayer]

An rcd moniters current on the outgoing live and the incoming neutral. if earth currents occur , the current on the outgoing live and the incoming neutral will differ . eg . 4A out on live - 1 amp down earth - 3 amp down neutral = imbalance. ie. earth leakage.

In reality the rcd operates when the difference is more than 30mA . ( i think ) .

An MCB works on the same principle as a fuse. When the current exceeds the safe capabilities of the cable . The MCB trips. Such as a short ( no load : high currents ) .

i think ...... :-D

 

[Deleted member 26818]

The thing about these devices, is that they will both operate when there is no fault.
MCBs provide overcurrent protection. That overcurrent could be caused by a fault such as Line to neutral or Line to earth, or it could be caused because too many high power appliances are plugged into a socket circuit.
RCDs detect earth leakage. This could be caused by a fault between a live conductor and earth, or could occur in the normal operation of an appliance.

 

[daiplayer]

Just a note on summat i learned recently , ever had the rdc trip when the mcb is off ? ( ie touching the n + e together ?)

Learned that small voltages on the neutral bar creates currents in the earth . and rdc trips.

Baffled me for ages that. lol

 

[andybeach]

Just a note on summat i learned recently , ever had the rdc trip when the mcb is off ? ( ie touching the n + e together ?)

Learned that small voltages on the neutral bar creates currents in the earth . and rdc trips.

Baffled me for ages that. lol

First time it happened to me I thought the house was haunted, lol.

The neutrals are connected together at the CU. So even though you have locked off the CB on the circuit you're working on, if you short the neutral to earth while working on it,it gives an alternative path for the current flowing in the neutrals of the other circuits. That causes the RCD to trip.

 

[daiplayer]

I thought i was going mad ..... thought the circiuts were all interconnected...lol