UK RCD vs MCB trip

[Beeg]

Evening everybody,
I’m wondering if someone wouldn’t mind answering a question for me about RCDs.

I had a water pipe burst behind my washing machine this evening, and once I’d isolated the leak I noticed the sockets had tripped. When I went to reset the breaker, I noticed that it was the RCD that had tripped, and not the MCB for the Ring Final Circuit which the washing machine is plugged into.
My question is ; why would the RCD trip but not the MCB? I understand that the MCB is there to protect the cabling in the event of fault current/overload, and that the purpose of an RCD is to interrupt the supply to reduce the effects of an electric shock. Would the RCD have tripped due to an earth fault such as the casing of the washing machine becoming live or water entering the socket behind the machine?
Thanks in advance and apologies for the lengthy post.

Regards

 

[Lucien Nunes]

Yes, water has got in somewhere and created leakage path(s) to earth. Plain water is conductive enough that a wet fitting could have sufficient earth leakage for the RCD to detect. However, to trip the MCB would require a leakage path in the order of 500 - 1000 times lower resistance, which probably would not occur even with a socket outlet submerged in water.

 

[dmxtothemax]

with two units in a series arrangemant the fastest one trips first,
once that has tripped then there is no reason for the second one to trip
so it's purely a speed issue.

 

[westward10]

with two units in a series arrangemant the fastest one trips first,
once that has tripped then there is no reason for the second one to trip
so it's purely a speed issue.

No it's a case if the water not producing a low enough resistance for fault current to flow, enough to trip the circuit breaker.

 

[DefyG]

with two units in a series arrangemant the fastest one trips first,
once that has tripped then there is no reason for the second one to trip
so it's purely a speed issue.

Sorry dmx gotta disagree. They are 2 different devices offering 2 different functions. RCD provides earth leakage protection and a MCB overcurrent protection (Whether overload or short circuit).

 

[Beeg]

Yes, water has got in somewhere and created leakage path(s) to earth. Plain water is conductive enough that a wet fitting could have sufficient earth leakage for the RCD to detect. However, to trip the MCB would require a leakage path in the order of 500 - 1000 times lower resistance, which probably would not occur even with a socket outlet submerged in water.

Thanks Lucien, will it be okay to leave the appliance/ socket to dry for 24hrs and the plug back in and try using it?

 

[dmxtothemax]

perhaps the leakage currant to ground during the fault was past the RCD threashold which is only >30MA but less than the mcd device which wil be > 20A.

 

[Kidsolo]

No it's a case if the water not producing a low enough resistance for fault current to flow, enough to trip the circuit breaker.

Woah, not so fast my friend. There is water involved, so there is going to be an earth fault. The reason the RCD trips first is because it is designed to trip within 40ms of a fault occurring.... way faster than any breaker or fuse will ever respond.

 

[dmxtothemax]

There was an earth fault ,thats why the RCD tripped,
But if the fault currant did not exceed the trip threashold of MCD,
Then this would explain why it did not trip.

 

[Kidsolo]

There was an earth fault ,thats why the RCD tripped,
But if the fault currant did not exceed the trip threashold of MCD,
Then this would explain why it did not trip.

NO, A MCB will trip with an earth fault, but not when it is also protected by a RCD as the RCD trips too fast.

 

[dmxtothemax]

so you think it is a speed issue rather than a currant threashold issue ?

 

[fairlight]

NO, A MCB will trip with an earth fault, but not when it is also protected by a RCD as the RCD trips too fast.

An mcb will require a much higher earth fault current to operate than an RCD. An RCD will only require a fault current of milliamps to operate whereas and mcb will require tens of amps over it's rated current, a current a 'wet' fault is unlikely to generate. However, yes, RCD trip times are faster than mcb times unless very high fault currents are involved.

 

[Beeg]

Thanks for the responses guys. I understand that once the RCD had tripped the MCB wouldn’t trip, however what is the theory behind the RCD tripping? Would it be that the water had shorted between L-E due to its high conductivity?

 

[dmxtothemax]

pretty much !

 

[Intoelectrics]

NO, A MCB will trip with an earth fault, but not when it is also protected by a RCD as the RCD trips too fast.

An mcb operates in 2 ways, thermal overload and high current short circuit.
If the earth leakage path resistance is to high then then there will be insufficient current to trip the short circuit mechanism of the mcb. The mcb will only trip if the earth leakage current is high enough to operate the short circuit mechanism. Or if the thermal mechanism experiences a high enough excessive current flowing for a long enough period of time (depending on the type and rating).
A rcd however is designed to monitor current imbalances between L & N, so a much lower resistance earth leakage path will be enough to create a detectable imbalance (sensitivity depends on the rating of the rcd) to operate this device.
So in the op's scenario the small earth leakage caused by the water created a low enough resistance path for there to be a detectable imbalance for the in service rcd to operate first. If there had have been a "short circuit" then the mcb could quite possibly operate before the rcd.