RCD tripping without ground fault

Baughn

Come closer, and listen to my tale of woe.

We have a 30mA RCD. Specifically not an RCDO; it should not trip purely due to load. It has been tripping a lot lately, however, and the local electrician has been somewhat unhelpful.

In a last-ditch attempt at diagnosing the problem, I disconnected all electrical devices, intending to gradually plug them back in, waiting a few hours between each to see what would happen.

I didn't get very far.

Switching on both tea cookers (a 4.5kW purely resistive load) caused the RCD to trip. They don't do so individually, but the combination reliably trips it. So, thinking there might be a ground fault in the cookers, I modified an extension cord to disconnect the ground wire. I know, terribly unsafe, etc., but it was safe enough in this case; they're made of plastic, and I used a plastic spoon to turn them on anyway.

Subsequently the RCD tripped. The breakers didn't, and it's not like I overloaded the circuit or anything; the living room is on a 20A breaker, which I was admittedly tickling the limits of, but the breaker didn't trip. The RCD did.

Any ideas what might be going on? To my limited understanding of electricity this shouldn't happen.

Zackdickensdog

Sounds like a ground fault on the circiut rather than the appliance, you would need to do an insulation test to confirm this

Baughn

Ah, but if there's a ground fault on the circuit, why does it only trip under load?

More importantly, does anyone know a decent electrician in the Dublin area who actually has a chance of fixing this?

tomdempsey200

Baughn wrote: »

Ah, but if there's a ground fault on the circuit, why does it only trip under load?

they can do-might be a circuit fault

test your cookers-1minute job

test rcd and final circuits

fit rcbos or second rcd if all else fails

Cycling Dumbasses

So, thinking there might be a ground fault in the cookers, I modified an extension cord to disconnect the ground wire

An RCD measures the current balance between the live conductor and neutral conductor through a differential current transformer, if there is an imbalance the RCD trips, disconnecting the ground wire would still not rule out a ground fault, i would carry your tea cookers to an appliance engineer for professional testing/repair, that experiment you did potentially was dangerous.

tomdempsey200

Cycling Dumbasses wrote: »

i would carry your tea cookers to an appliance engineer for professional testing/repair.

no need
switch 'em on and put the megger on them
earth cont. and ins. resistance
tells you all you need to know here

Baughn

Cycling Dumbasses wrote: »

An RCD measures the current balance between the live conductor and neutral conductor through a differential current transformer, if there is an imbalance the RCD trips, disconnecting the ground wire would still not rule out a ground fault, i would carry your tea cookers to an appliance engineer for professional testing/repair, that experiment you did potentially was dangerous.

It's not going to explode.

Could you explain, though, because perhaps I'm missing something - how can the cooker cause an imbalance between live and neutral when only live and neutral are connected? It was sitting on a wooden floor, there's nowhere else for the current to go.

tomdempsey200

N-E fault on circuit wiring causes it

Baughn

Ah. All right, that matches my research; I wanted confirmation, but I didn't want to bias your answers.

Now let's see if I can find an electrician who can do something about it.

Cycling Dumbasses

Baughn wrote: »

It's not going to explode.

Could you explain, though, because perhaps I'm missing something - how can the cooker cause an imbalance between live and neutral when only live and neutral are connected? It was sitting on a wooden floor, there's nowhere else for the current to go.

You need to look at the bigger picture the cookers could be causing an imbalance in the supply circuit, a cable may be breaking down i.e low insulation value, an RCD wont normally trip if no load is going through it even though it may have more than one supply circuits with several cables of low insulation value, if you had low insulation value in your supply circuit i.e cable leaving consumer to socket, you will get leakage which in turn trip out RCD.
If you plug the water heaters into a different circuit and they trip the RCD once nothing is plugged into any socket in the living room you will eliminate the possibility of a problem in your living room circuit, i believe they will trip the RCD and you have a problem with one of the water heaters, e.g a loose or bad connection, break down in element in the heater which can cause RCD to trip, you could double confirm if you tried the heaters in another house, as said in last post i would get water heaters to a professional appliance engineer if you eliminate they are at fault.
Remember an RCD can trip out for several reasons.

Bruthal

Properly working RCDs usually only trip out on imbalance.

In the OPs case, because it is taking both together to trip the RCD, and he has tested it with the earth disconnected, its likely there is a neutral to earth fault on a circuit controlled by the RCD.

If it is a direct short from neutral to earth, it is likely on a circuit other than the one they are being plugged into.

Bruthal

Cycling Dumbasses wrote: »

an RCD wont normally trip if no load is going through it even though it may have more than one supply circuits

That is not always true. A neutral to earth direct short on a socket circuit only needs to divert 0.03 amps from the main neutral bar, through the RCD neutral, to the short on the earth, to trip the RCD, without any load on the RCD circuits.

It usually requires a high load on the non socket RCD circuits to happen, but it does happen.

A neutral to earth short downstream of the RCD also makes an RCD less sensitive, and therefore it wont give as much shock protection under such circumstances.

Cycling Dumbasses

My previous post with my view in relation to an example of a RCD tripping was due to low insulation value, i did not mention a direct short circumstance in my last post. I did not offer any opinion on an RCD trip with direct short.

Bruthal

You realise I'm sure, that a Circuit direct short, and a neutral to earth short, are different things.

If the insulation in a cable was so bad that it trips an RCD, nothing would need to be plugged in to cause such trips. The minimum live to earth impedance value needed to trip an RCD would be far higher than the neutral to earth value needed, and with neutral to earth fault, a load usually needs to be on, to cause trips.

Therefore, a cable bad enough to cause such events, will be tripping on live to earth faults, with nothing plugged in. The op case happens with items being plugged in.

Cycling Dumbasses

Cycling Dumbasses wrote: »

My previous post with my view in relation to an example of a RCD tripping was due to low insulation value, i did not mention a direct short circumstance in my last post. I did not offer any opinion on an RCD trip with direct short.

Sorry to clarify, direct short as in neutral to earth.

Bruthal

Cycling Dumbasses wrote: »

Sorry to clarify, direct short as in neutral to earth.

Yes, but the op describes RCD tripping when cookers are plugged in.

And says earth's are disconnected. Likely problem is neutral to earth fault or short in a circuit from the RCD .

Bruthal

Anyway no debates anymore, or the 242 comes out.

Cycling Dumbasses

Bruthal wrote: »

Anyway no debates anymore, or the 242 comes out.

I am not having a debate with you, i dont wish to entertain into a debate with you, all i ask is if you read my posts properly and if you quote a reply from them keep it relevant to what i said, that is all.

Bruthal

Cycling Dumbasses wrote: »

I am not having a debate with you, i dont wish to entertain into a debate with you, all i ask is if you read my posts properly and if you quote a reply from them keep it relevant to what i said, that is all.

Your posts are simple to read. You told the op bad insulation would trip RCD. I think that's unlikely.

Cycling Dumbasses

Bruthal wrote: »

Your posts are simple to read. You told the op bad insulation would trip RCD. I think that's unlikely.

I said:

a cable may be breaking down

Why cant you take the time to read posts in full. You are entitled to your opinion and im entitled to mine, i did not go slating your opinions.

Bruthal

Cycling Dumbasses wrote: »

I said:

Why cant you take the time to read posts in full. You are entitled to your opinion and im entitled to mine, i did not go slating your opinions.

I slated nothing. I said RCD tripping due to bad insulation is unlikely.

And even if it was due to cable insulation deteriorating, live to earth current would cause trips far earlier than neutral to earth current, and no appliances need to be plugged in to cause that. Current flow to an appliance won't really affect how much current leakage there is in cable insulation.

Cycling Dumbasses

Whatever the cause it would be interesting for OP to give us an update of what the problem was.

Bruthal

@Baughn . you were correct in assuming the appliance should not trip the RCD with its earth disconnected.

The usual cause is a neutral to earth fault in the installation in such circumstances, assuming there is no other earth path from the appliance, such as the appliance sitting on sink draining board etc, and appliance itself has earth fault.

Bruthal

Cycling Dumbasses wrote: »

Whatever the cause it would be interesting for OP to give us an update of what the problem was.

Knowing my luck, it will be faulty cable insulation:)

Cycling Dumbasses

Im putting it down to one of the heaters being faulty, be interesting to hear this outcome.

Bruthal

Cycling Dumbasses wrote: »

Im putting it down to one of the heaters being faulty, be interesting to hear this outcome.

If the earths were removed from the appliances, and the metal frames isolated from any earthing, then a fault on the appliances can't really trip the RCD.

Cycling Dumbasses

Bruthal wrote: »

If the earths were removed from the appliances, and the metal frames isolated from any earthing, then a fault on the appliances can't really trip the RCD.

I have seen badly burnt/loose connections before on appliances trigger the RCD, just going off my own experiences.

Bruthal

Cycling Dumbasses wrote: »

I have seen badly burnt/loose connections before on appliances trigger the RCD, just going off my own experiences.

So have I, but only when The burnt connections also included the earth terminal beside them being affected, or affected some earthed part nearby.

How could burnt connections trip an RCD on an item which is isolated from the earthing in an installation?

Cycling Dumbasses

I cant answer that question, all i can say is i have seen it and with earth connection out of the equation, like you said earlier all it takes is 0.03 amps of an imbalance, could that imbalance come from arcing-bad connections trying to make? this is my only theory, what do you think?

The Glass Key

Bruthal wrote: »

If the earths were removed from the appliances, and the metal frames isolated from any earthing, then a fault on the appliances can't really trip the RCD.

The current on both the live and neutral has to be the same (within ~ 30mA) or the RCD will trip. You seem to be getting this mixed up with earth leakage trip.

The most obvious way for the current on the live and neutral to have an imbalance is for the current coming down the live wire to be shorted to earth often via the earth cable but that's not the only way. If we have no earth and you were to touch the live wire you'd be sending current to earth through you and there'd be an imbalance between the live an neutral and the trip would go off.

Now if you have say some damp in the walls and some damaged cable in that area that could also create an imbalance but if it creates an imbalance of say 25mA then the trip won't go off. Now if you switch on an appliance that draws a lot of current then the conductivity of the damp patch may change or if the appliance itself might create a small imbalance of 6mA, normally this 6mA wouldn't trip the trip but with an existing wiring problem it can make it look like the problem is the appliance when it might not be.

If I have that all wrong I'm sure some will be along soon to put me right

Bruthal

Any arcing would be balanced if there is no earth fault on the appliance. Often seen arcing at sockets from worn sockets, without RCD trips.

Shower pull chords frequently reported burning out. No RCD trips occur with them.

Cycling Dumbasses

Bruthal wrote: »

Any arcing would be balanced if there is no earth fault on the appliance. Often seen arcing at sockets from worn sockets, without RCD trips.

Shower pull chords frequently reported burning out. No RCD trips occur with them.

I be of the opinion arcing may not be balanced, this is my only explanation really, i see your point on the shower pull cord switches.

Bruthal

The Glass Key wrote: »

The current on both the live and neutral has to be the same (within ~ 30mA) or the RCD will trip. You seem to be getting this mixed up with earth leakage trip.

Very good. Except when an imbalance occurs, its due to reduced current return on neutral. Where do you think that residual current goes?

The most obvious way for the current on the live and neutral to have an imbalance is for the current coming down the live wire to be shorted to earth often via the earth cable but that's not the only way. If we have no earth and you were to touch the live wire you'd be sending current to earth through you and there'd be an imbalance between the live an neutral and the trip would go off.

Great. But read the op. Item plugged in, isolated from earth.

Also, a person touching a live terminal will also need to touch something earthed to trip an RCD. I've done it myself a couple of times by accident. Its a fairly nasty shock before it trips. It won't trip by a person simply contacting a live item alone.

Now if you have say some damp in the walls and some damaged cable in that area that could also create an imbalance but if it creates an imbalance of say 25mA then the trip won't go off. Now if you switch on an appliance that draws a lot of current then the conductivity of the damp patch may change or if the appliance itself might create a small imbalance of 6mA, normally this 6mA wouldn't trip the trip but with an existing wiring problem it can make it look like the problem is the appliance when it might not be.

If I have that all wrong I'm sure some will be along soon to put me right

I'm sure its all perfect

Bruthal

Cycling Dumbasses wrote: »

I be of the opinion arcing may not be balanced

Arcing on a circuit will be balanced. If not, then every time we plug any item in and switch on, the RCD would trip.

Some arcing always occurs at switching on and off of items. And where is the arcing current flowing, only on L & N.