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

 

[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.
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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!