UK RCD vs MCB trip

[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.
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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.