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

[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?

 

[needasparks]

Correct, I think it used to be called a sensing coil that the flux is created by but its been a while since i've I was at college so I;m sure someone will correct me,

 

[Kev2632]

Correct, I think it used to be called a sensing coil that the flux is created by but its been a while since i've I was at college so I;m sure someone will correct me,

yea im beginning to understand now what you mean, so when the faulty neutral was taking ouf of the neutral, the current flowing in the neutral live are now the same hence why it is allowed to trip on the test button is that right Needasparks?