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

 

[Kidsolo]

RCD trip times are faster than mcb times unless very high fault currents are involved.

I would still expect the RCD to trip first

 

[Lucien Nunes]

In the case of short-circuit I don't think anyone is arguing specifically against that, but it's not relevant here as there wasn't a hard short. The socket outlet and the washing machine got sprayed with water, which is not conductive enough for a wet fitting to leak 46A (1.45 x 32) between a couple of small bits of brass and copper. The leakage probably ramped up slowly from nothing as the water penetrated.

OP: It would be best to get the circuit and the machine tested for insulation resistance before powering back up, to avoid possible nuisance tripping or further damage. If the machine's guts are soaked, aside from RCD tripping, permanent damage to the programmer or motor controller / inverter may be caused by trying to operate it while they are still wet. In a hurry, I would change the plug and socket, then test the machine.

so a much lower resistance earth leakage path will be enough to create a detectable imbalance
Source URL: UK - RCD vs MCB trip - https://www.electriciansforums.net/threads/rcd-vs-mcb-trip.179693/page-2

Your explanation is good, but I think you meant higher, not lower!

 

[Intoelectrics]

In the case of short-circuit I don't think anyone is arguing specifically against that, but it's not relevant here as there wasn't a hard short. The socket outlet and the washing machine got sprayed with water, which is not conductive enough for a wet fitting to leak 46A (1.45 x 32) between a couple of small bits of brass and copper. The leakage probably ramped up slowly from nothing as the water penetrated.

OP: It would be best to get the circuit and the machine tested for insulation resistance before powering back up, to avoid possible nuisance tripping or further damage. If the machine's guts are soaked, aside from RCD tripping, permanent damage to the programmer or motor controller / inverter may be caused by trying to operate it while they are still wet. In a hurry, I would change the plug and socket, then test the machine.



Your explanation is good, but I think you meant higher, not lower!

yes, it was a mistake, I tried to edit it but for some reason I couldn't. Thanks

 

[DefyG]

It’s not a race RCD -V- MCB, the RCD is there to detect earth leakage current and will operate within 40mS if a leakage current ‘exceeding’ 30mA exists.
In the situation described it would appear that an earth leakage current has caused the RCD to operate.
A MCB May permit, for example 3-5 times the rated current to flow before it operates with a much longer permitted disconnection time (typ 0.4s [400mS] if TN-S.
A short circuit current will reduce this time but that doesn’t sound like the fault described.

 

[Beeg]

Thanks everyone for your replies. I took the back off the washing machine and there didn’t seem to be a lot of water in there. I replaced the socket and plugged it in and all seems to be working fine

beeg

 

[Kidsolo]

but it's not relevant here as there wasn't a hard short. The socket outlet and the washing machine got sprayed with water, which is not conductive enough for a wet fitting to leak 46A

Hold on a minute, are you trying to tell me that you wont get a dead short from water? and it doesn't have to be 48 amps, as the circuit itself would already have a load on it that is at near full capacity.
What do you think we relied on before RCD's?

You crazy UK leco's should brush up on both your practicle and theory.

 

[DPG]

On the evidence I've seen, Lucien's electrical theory knowledge is way ahead of you kid.

 

[DPG]

Hold on a minute, are you trying to tell me that you wont get a dead short from water? and it doesn't have to be 48 amps, as the circuit itself would already have a load on it that is at near full capacity.
What do you think we relied on before RCD's?

You crazy UK leco's should brush up on both your practicle and theory.

A bit of damp from tap water does not create a dead short, no.

 

[Lucien Nunes]

are you trying to tell me that you wont get a dead short from water?

Yes. Plain water is not very conductive, specific conductivity of tap water is in the order of 10mS/m at room temperature. If the device remains energised, especially with DC, the conductivity will increase with the concentration of metal ions or TDS due to electrolysis of the metal parts.

For what it's worth, certain high-power electronic devices using water cooling actually place the water in contact with energised parts, but the water is kept pure enough to achieve a conductivity of just a few μS/m so that over a long insulated tubing path it is effectively an insulator.

What do you think we relied on before RCD's?

Earthing. The water in the fitting would have gently heated and fizzed and boiled itself off, leaving a working fitting but possibly with impaired IR due to a film of salts deposited on the surface and/or metal tracking.

 

[Intoelectrics]

Hold on a minute, are you trying to tell me that you wont get a dead short from water? and it doesn't have to be 48 amps, as the circuit itself would already have a load on it that is at near full capacity.
What do you think we relied on before RCD's?

You crazy UK leco's should brush up on both your practicle and theory.

Ermm... a dead short is a path of low resistance allowing a high current flow.
It has been suggested and most likely, that the water spray did not produce a low enough resistance to allow enough current to pass to trigger the fault current device. But rather, as in this case, enough of a leakage to earth to create an imbalance and thus trip the rcd.

The washing machine is unlikely to be much higher than 10-12 amps at full load so only a 1/4 of the fault current required to trip the mcb under fault conditions. Its unlikely that the fault leakage path created by the water would have sufficiently low enough resistance to achieve the required fault current to trip the mcb

So in the case of the OP scenario it was indeed an earth leakage fault.

 

[Kidsolo]

On the evidence I've seen, Lucien's electrical theory knowledge is way ahead of you kid.

Apparently not.

Plain water is not very conductive

Throw a hairdryer in the bathtub and see how conductive it is.

If the device remains energised, especially with DC

When did DC come in to it? you can't change the story to suit your theory.

The water in the fitting would have gently heated and fizzed and boiled itself off, leaving a working fitting

You have a nice way with words.... the only one you left out was "gently massage"

Its unlikely that the fault leakage path created by the water would have sufficiently low enough resistance to achieve the required fault current to trip the mcb

Facts don't work on something being unlikely, but it was worth a shot

OP: It would be best to get the circuit and the machine tested for insulation resistance before powering back up, to avoid possible nuisance tripping or further damage. If the machine's guts are soaked, aside from RCD tripping, permanent damage to the programmer or motor controller / inverter may be caused by trying to operate it while they are still wet.

Why? apparently "
The water in the fitting would have gently heated and fizzed and boiled itself off, leaving a working fitting"
Source URL: UK - RCD vs MCB trip - https://www.electriciansforums.net/threads/rcd-vs-mcb-trip.179693/page-2#post-1583545

 

[DPG]

Kidsolo - your response is poor and juvenile. I advise you to read some of Lucien's posts on other threads on this forum - you will find his level of technical knowledge very impressive, and also his attitude much more commendable than yours.

Stay on the forum though - you may well still be able to prove your worth if you try.

 

[Midwest]

At my work place, the RCBO had tripped on some outdoor LED battens. They had failed, and slowly filled up with rainwater, and eventually tripped it. Think it tripped, as the rainwater had nowhere else to go. ?

 

[Lucien Nunes]

I tried that hairdryer and bath thing but all I got was a small crack in the casing of the hairdryer. It still doesn't seem very conductive. Should I put some water in the bath and try again? Maybe try throwing the washing machine in as well for good measure?

BTW I'm out... I've got customers here!

 

[Kidsolo]

I advise you to read some of Lucien's posts on other threads on this forum

I will get to his other posts, but I just want to correct him on this subject first, Boss.