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.

 

[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.

 

[Strima]

Water is a very poor conductor, it's the deposits such as minerals which will increase its conductivity.

 

[Intoelectrics]

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

Then present some yourself.
The Op asked why the rcd had tripped and not the mcb. We have successfully answered the question.
The reason I used the word "unlikely" is because in this scenario it was unlikely. I never said impossible. After all it's down to Ohms law and the characteristics of the protection device which determines the trip thresholds. The fact is that there was an imbalance detected before a short circuit developed, it was the rcd that had tripped.
[automerge]1571920786[/automerge]

Water is a very poor conductor, it's the deposits such as minerals which will increase its conductivity.

Yes, and its conductivity depends on the type and amount of impurities it contains. Sea water for example is far more conductive than tap water.

 

[Kidsolo]

The fact is that there was an imbalance detected before a short circuit developed,

because a RCD reacts faster than a breaker. I said that in my very first post.
You can't say for sure that if there wasn't a RCD that the breaker wouldn't trip.

 

[David O'Connell]

Had a similar fault the other day with a microwave oven tripping the 30mA RCD on my kitchen ring. My 15yr old daughter had placed a bowl of boiling water into the microwave, to cook some pasta, which had then overflowed and collected around the bottom, shorting out the motor that spins the glass plate around. I had to give my daughter a severe reprimand, drain all the water out of microwave and leave it in the airing cupboard to dry out for 24hrs. It's worked no problem after that. I tested the RCD afterwards and found out that it trips at 21mA, far less than the 160A required to trip the circuit breaker. Hence why my circuit breaker didn't operate either.

 

[DPG]

But don't forget, the RCD will trip on N-E faults also, regardless of speed issues.

 

[Kidsolo]

the 160A required to trip the circuit breaker

don't worry, it would blow the guts out of the micromave before tripping your breaker with 160 amps. You might want to reassess your breaker ratings.

 

[Intoelectrics]

because a RCD reacts faster than a breaker. I said that in my very first post.
You can't say for sure that if there wasn't a RCD that the breaker wouldn't trip.

Ermm... No, not always, depends on the type & characteristics of the fault. (my bold)

I can't recall stating that the mcb would never trip, again this depends on the characteristics of the fault. If there is insufficient fault or overload current to achieve the trip threshold characteristics of the protection device then it will not trip.

Lucien Nunes has explained all this, you have yet to show any evidence to back up your claim that Lucien is wrong.

 

[plugsandsparks]

This is the way i have always understood it, and experience appears to support.

On a L-E fault, RCD will always trip first, it has to cos its got a 30mA limit and a 40mS time to hit, i dont think you can buy a MCB that would trip faster.
On a L-N fault , MCB always trips first, RCD sits happy
On a N-E fault, MCB never trips, RCD can and thats when you start chasing your tail finding the blessed fault, lol

 

[littlespark]

I'm glad the OP got the question answered. re the difference between MCB and RCD. Thread slightly wandered off at a tangent for a bit though.

On a side note... PLEASE don't throw a plugged in hairdryer into a bathtub of water to see how conductive it is.

 

[Lucien Nunes]

A long time ago, in a country far far away (in N. Africa) I was working on a project based at a camp in the desert. One night there was a violent storm that flattened many of the marquees and caused a lot of damage, Equipment and supplies got drenched, buried in the sand or both. We recovered quickly, but during the downtime as I couldn't do my usual job, I tried to help people get their appliances up and running again. Time was too short for a detailed strip down of anything that did not strictly need it, so the task was really to get the sand out of stuff and then let the scorching late morning sun take care of the damp. We set up some dustbins full of water to wash the worst of the sand off the appliances ready for triage.

One guy came over with a dozen hairdryers / curling tongs / straighteners full of wet sand. Somewhere, there exists a picture of me holding them hanging on their flexes, swooshing them about in the water. Most were up and running within about 10 minutes, plus or minus a gritty-feeling switch or two. The only other pic of me from that project that I know exists shows me fast asleep in a deckchair, two days later when my own specific disaster had played out. That story is much too long to post here though, suffice to say it involved a lot of sand.

BTW, no we were not vain or daft for wanting hair styling equipment in the desert!