Unusual(?) RCD trip

[tony sayer]

In article , tony sayer scribeth thus

Correction!!!

Earth to Neutral short!!!!

[tony sayer]

Yep but their phase to neutral not Earth?..

Just how much electronic stuff are you running on the one 30 ma trip NP?...

[The Natural Philosopher]

Its 100mA now.

2 Mac G4's 2 PCs. 2 powered speakers for 2 machines. PABX laser printer DSL router Ethernet switch TV distribution amp. 3 set top boxes. 3 TVS on standby. Possibly 2 more not. One Hifi setup often on standby. Scanner Print server box. two home chargers and camera battery charger. three radios. Often a 12v bench PSU. nd two trickle chargers. Radio thermostat. Oil level monitor. The odd other wall wart for something obscure like recharageable torches..

Then add in a fridge, two fridge freezers and a chest freezer.. plus an oil boiler, autocycling water softener, and dishwasher and washing machine..

Nightmare innit?

[Harry Bloomfield]

Andy Burns formulated on Sunday :

I don't think anyone has given Andy an explanation of just how RCD's work....

Basically there are two current sensing coils, one sensing the current passing through the neutral and the other the current passing through the live conductor. The two are compared for any difference (high or low) in the two current levels and any difference indicates a leakage. If the leakage is assumed to be more than a preset level, then it trips out. Any small amount of leakage (30mA?) from live to earth or from neutral to earth, is enough to cause the trip.

[Kevin]

nearly right there are two coils in most units but they are wound in opposition through a transformer and the current flowing through one cancels out the other, no need for fancy comparison circuits see

[Harry Bloomfield]

Kevin has brought this to us :

[Andy Burns]

The bonding to gas is close to the CU, both of which are just through the outside wall to the electricity meter and main earth point. When I discovered that the bonding to the water had been severed, I had to take a longer route to reinstate it (about 10m of 16mm^2) and the "far" end of that is close to the kitchen ring which is the suspect.

[Andy Burns]

Thanks, I know more or less how they work; I'm not puzzled at how/why it tripped, if the fault persisted I'd expect the RCD to refuse to reset, but I am puzzled at why turning off/on the particular MCB then allowed the RCD to reset?

[tony sayer]

Some of that won't have caps to earth, across the line but not earth..

Not much different to what we run here and trips are very, very, rare perhaps once every two years or so and thats on a 30 ma...

We've got some comms sites with a lorra filtering all on 30 ma trips perhaps now and again in a direct or near miss lightning strike;!..

[The Natural Philosopher]

Cos you prolly had a BIG capacitative live-to-earth load on that one.

[John Rumm]

The RCDs (these days anyway) usually have a certain amount of filtering to reduce the effect of capacitive coupling of transient related noise and harmonics. IME you don't usually get trips on transients unless the device is already sensitised to some extent.

Pull the main fuse...

FWIW, when I moved in here at the start of the year, there were 10 circuits all sharing a (oldish) 30mA RCD (including feeds to outside sockets and outbuildings etc). We had a couple of trips out of the blue, and then quite a number on transients (every thunderstorm, bulbs blowing on so on) would also cause trips. So I ripped the lot and did a 16th edition style TT setup, with 100mA time delayed RCDs in two split load CUs (one for indoor and one for exterior / outbuilding circuits[1]). With the 30mA RCD only covering the 3 main power circuits. Not had a single RCD related problem since.

[1] That's the theory, however I still have to disentangle the outside sockets and fish pond pump from the upstairs ring final circuit!

[John Rumm]

Impedance to earth from where exactly? Not sure I follow.

The thing to bear in mind here is that the potential of the neutral wrt to earth is only of relevance in the presence of a wiring fault (i.e. neutral to earth short). The reason these are more "interesting" than line to earth shorts is that they can go unnoticed some of the time.

[Kevin]

simplifying by inventing something thats not in it ????