Grant External Combi Boiler that wouldn't switch CH off.

[CER]

I've already part solved this problem. It took a while but I eventually tracked the problem down to the fact that the exterior isolation switch had been letting in (and collecting) water. This then was in some way making the switched live wire into a permanent live wire. However my question is would you have expected this circuit to trip with the existence of water in the external switch? Its a simple circuit, 16amp circuit breaker, Internal isolation switch to a wireless thermostat receiver, external isolation switch to the Combi grant boiler.
Many Thanks.

 

[Echo the husky]

There should be a 5A fuse somewhere!

 

[stillp]

Is the external switch metallic, and earthed? If so then I would have expected your RCD to trip. There's no way there would be enough current to trip the MCB.

 

[CER]

There is a 3A fuse in the internal isolation switch. The external switch is plastic, and earthed.

 

[stillp]

What part of the plastic switch is earthed? A photo would help. Is the circuit RCD protected?

 

[CER]

This is the external Isolation switch. As a temporary measure i have removed the swich, and connected wires using a terminal block and then wrapped in tape. Two things spring to mind. 1) Even though the electic supply comes from above, why not enter and exit the wires from beneath to stop the possibility of water entering at the top of the box. 2) Why are the earth wires in the N terminal and not the ones with the earth symbol.

1

• CER
• 17 Feb 2016

External Isolation Switch

This is the damaged switch i removed.

4

• CER
• 17 Feb 2016

Dodgy switch.

This is the thermostat wiring, and isolation switch

1

• CER
• 17 Feb 2016

Thermostat wiring.

2

• CER
• 17 Feb 2016

Thermostat isolation swich

This is the switch board.

3

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• 17 Feb 2016

Fuse box

Many Thanks.

 

[stillp]

1) Even though the electic supply comes from above, why not enter and exit the wires from beneath to stop the possibility of water entering at the top of the box.

Maybe the installer wan't the cleverest. Yes, the cables should enter from the bottom. Is htere a drain hole?

2) Why are the earth wires in the N terminal and not the ones with the earth symbol.

As above. However, it shouldn't matter. The earth terminal looks as if something was connected to it though.

The switch you removed looks as if it's been overheating, as well as full of water.

Does that RCD FCU (fused spur) trip when you press the "Test" button? It might not have tripped when the switch was full of water, if there was less than 30mA flowing to earth. The N terminal doesn't look as if it was under water, so maybe there was no leakage to earth, so no current imbalance to trip the RCD.

 

[CER]

Yep, the RCD FCU trips when i press the test button. The reason for my original question was to try and determine if there was something else wrong wrong with the circuit other than the switch. I therefore thinking that i will just do a straight replacement on just the external isolation switch (and re-route the entry point wire )
Many Thanks.

 

[CER]

Just a follow on question. The MCB for the boiler circuit is 16A, and the previous rotary isolation switch was 20A. Should I change the rotary isolation switch to 16A?
Thanks.

 

[stillp]

No need. The 20A rating is just the maximum the switch can carry.

 

[Taylortwocities]

Should I change the rotary isolation switch to 16A?

As Stillp says, no need.

As an analogy. You buy a car and the speedo dial can go up to 150mph. The maximum speed that the car can travel is 100mph. Would you need to change the speedo for one that only goes up to 100mph?