Quick question about electrics, RCD etc.

[JP_]

Trying to understand this, as it really baffles me. In short, what does the RCD do? Background to question:

The other night our power went off momentarily and tripped the RCD. I could not put the RCD back up - something was forcing it to stay off. So I turned off all sockets and the RCD then worked again. But, have since turned all back on again, and no problem. All the while, the lights all stayed on - so lights not on same circuit as RCD? That I get. This is where it gets weird.

A neighbour sent an SMS after the power cut with the same problem (she lives alone and was worried I think). Power cut, then sockets not working, but lights OK.

Now, I assumed that the power cut / surge (whatever it was) upset one of my appliances, but after turning everything off and on again (OK, not everything, I didnt turn off the central heating controls, but everything else in all rooms was turned off) it was OK.

But, if a neighbour had exactly the same problem, this suggests that not an appliance (unless a coincidence?). Houses were built at the same time, so have same fuse box / consumer unit.

I am just trying to understand what might have happened. Does the consumer unit have some sort of time delay in it? Or does turning sockets off lead to a reset? #confused

 

[JohnW2]

Trying to understand this, as it really baffles me. In short, what does the RCD do?

The short answer is that it detects small differences between current flowing the the Line ('live') and Neutral conductors and, if it detects such differences, disconnects the supply.

Normally, all the current flowing into something (like an appliance) will return to the consumer unit (and hence back to the supplier's substation) in the neutral, so L and N currents will be identical. However, if a 'leak' to earth arises (from L or N), at least some of the current will return via earth, not neutral. The N current going through the RCD will therefore be less than the L current. That difference will be detected and will make the RCD trip, disconnecting the power.

RCDs are very sensitive. They typically will detect a L-N difference of 30mA (0.03A) even when the total current flowing (through L and N) is, say, around 30A. That is sensitive enough for the RCD to trip if, say, water gets into the electrics and provides a path to earth.

So, in essence, RCDs detect a 'leak to earth' and disconnect the supply if one occurs.

Does that make sense?

Kind Regards, Johhn

 

[JP_]

Thanks John, that does make sense.

So, why would a RCD refuse to return after a powercut, but be OK after sockets are turned off and then, on again? And why would a neighbour be affected in the same way? It is likely that we both have different appliance issues causing a problem? But... if we do, what sort of appliance upsets the circuit, then stop upsetting after being turned off and on again?

 

[JohnW2]

So, why would a RCD refuse to return after a powercut, but be OK after sockets are turned off and then, on again? And why would a neighbour be affected in the same way? It is likely that we both have different appliance issues causing a problem? But... if we do, what sort of appliance upsets the circuit, then stop upsetting after being turned off and on again?

Unfortunately, it's not really possible to answer those questions. RCDs can be funny things, sometimes trip for reasons that are never discovered, can be upset by power cuts, lightning etc.

In general, the appliances most likely to cause 'genuine' RCD trips are those where electricity and water may come together - washing machine etc., immersion heaters, outdoor lighting etc.

All I can really suggest is that if this problem happens repeatedly that you get an electrician to check out your electrical installation. It can sometimes be very difficult to find the cause of intermittent problems, but (s)he might be able to dioscover what is going on.

Kind Regards, John

 

[EFLImpudence]

Some RCDs are like that. (Crabtree ?) They won't turn on again until the MCBs are off.

Not sure why, or how they do it.
I suppose it prevents continual forcing on of the RCD while the fault is still there.