RCD tripping using test button, N-E earth fault.

[Kev2632]

Hello, ive just been at a job today which i will be returning tomorrow which i have a problem with 2 Ring mains on the RCD,

*1 RINGMAIN 1
*2 RINGMAIN 2

Now when i switch on the RCD, and NO mcb's are on, the Test button doesnt work, and its the same when the mcb's are switched,

Then i tried disconnecting 1 of the neutrals from RINGMAIN 1, and put the RCD back on, the test button now works?

Why would this be happening? and is it a N-E fault, if so why doesnt the RCD trip straight away when the power is put onto the Circuits?

 

[Kev2632]

Does sound about right to me, Rcd's do funny things when a N-E fault is present and now you mention it wylex have done that to me on the last two i've tested. It won't trip on test button but will go randomly but when you test at x1 you will get a very fast trip time like less than 10ms. Any overly fast times normally that you would not expect to get usually ends up a N-E fault.
Am sure your IR test will confirm as tel says.

in that explanation what is meant by this

• (i) Along the neutral through the RCD coil
  
  or
• (ii) Along the neutral wire toward the load

what exactly is meant by this?

 

[needasparks]

Think you have to remember an rcd has two coils and its the current produced between them that operates. In theory if you have a N-E fault the neutral will have no potential and if no current is drawn on the live both coils will be at unity.

 

[Kev2632]

Think you have to remember an rcd has two coils and its the current produced between them that operates. In theory if you have a N-E fault the neutral will have no potential and if no current is drawn on the live both coils will be at unity.

yea i think i know what you mean...

could you explain a little further, as to what was meant by the two points i put on just before your reply there?

 

[needasparks]

It means in this state it becomes transient or floating even eliminating an actual load which is what is required to operate.
The term load referers to the current actually being produced which is an end product or resultant.

 

[Kev2632]

It means in this state it becomes transient or floating even eliminating an actual load which is what is required to operate.
The term load referers to the current actually being produced which is an end product or resultant.

and the reason this happens is, that the live comes into the rcd and trys to go through the coil, but why is this stopping the test button from working ?

 

[needasparks]

and the reason this happens is, that the live comes into the rcd and trys to go through the coil, but why is this stopping the test button from working ?

The transient state cancels out some of the fault, I don't know any planner way of explaining.

If i had a heavy weight in my hand I would drop it as it was too heavy. If you helped me carry it, i may not drop it as it is not as heavy. sorry not the best analogy (can hear tel laughing) but best i can do with a glass in my hand.

 

[Kev2632]

The transient state cancels out some of the fault, I don't know any planner way of explaining.

If i had a heavy weight in my hand I would drop it as it was too heavy. If you helped me carry it, i may not drop it as it is not as heavy. sorry not the best analogy (can hear tel laughing) but best i can do with a glass in my hand.

so in this case for my RCD, there is some current flow returning on the neutral and earth, but not enough to trip the rcd using the test button is that correct? sorry haha

 

[needasparks]

I'm going to have a lay down.............Where's me glass gone !

 

[Kev2632]

I'm going to have a lay down.............Where's me glass gone !

obviously im wrong en

 

[needasparks]

obviously im wrong en

No your correct, your trying to think too deep. Its easier than you think mate.
If its not tripping its because there is not enought fault current actually being seen by the rcd, hence why it becomes vague. you can press it 99 times and nothing happens, press it once and it may trip.

 

[Kev2632]

No your correct, your trying to think too deep. Its easier than you think mate.
If its not tripping its because there is not enought fault current actually being seen by the rcd, hence why it becomes vague. you can press it 99 times and nothing happens, press it once and it may trip.

Yea im sorry needaspark, its probz is easier than it looks, im always trying to think too hard when it comes to things like this, i just need to try and think it through easier ya know, just need to find an easier way of knowing it,

If you have time could explain a healthy circuit, and then a N-E Fault, as to why the button works ok on a healthy circuit and not when a N-E fault thanks.

 

[needasparks]

An RCD cannot create an fault earth path when you press the test button as it has no earth conection so it creates its own inbalance by reducing the induction on the neutral coil so for instance if both coils had 100 turns and i induced a voltage with a resistor partway on one coil (neutral) at say 50 turns it would unbalance the coils and trip. if some of this current became transient and some went through the neutral coil and some went via and earth path it may not generate enough flow for the device to operate, in effect cancelling itself out. conversly if the neutral path to the coil was a shorter route (when you disconnected the neutral) it would leave no other route so would trip.

 

[Marvo]

An RCD cannot create an fault earth path when you press the test button as it has no earth conection so it creates its own inbalance by reducing the induction on the neutral coil so for instance if both coils had 100 turns and i induced a voltage with a resistor partway on one coil (neutral) at say 50 turns it would unbalance the coils and trip.

With this mechanism the RCD test button would only work if there was a load current flowing at the time of testing.

 

[needasparks]

Hence why a resitor induces a current to flow. somewhat small but enough to operate under normal conditions.

 

[Kev2632]

An RCD cannot create an fault earth path when you press the test button as it has no earth conection so it creates its own inbalance by reducing the induction on the neutral coil so for instance if both coils had 100 turns and i induced a voltage with a resistor partway on one coil (neutral) at say 50 turns it would unbalance the coils and trip. if some of this current became transient and some went through the neutral coil and some went via and earth path it may not generate enough flow for the device to operate, in effect cancelling itself out. conversly if the neutral path to the coil was a shorter route (when you disconnected the neutral) it would leave no other route so would trip.

When you mention about me taking the neutral core out the neutral bar, and your saying that then means the current all returns too the RCD and return to the coil and it will operate the Button is that correct?