I'm baffled - Electric Velux Windows

[Desie]

Guys

This is a really strange one and at the moment I'm stumped. Please post as many answers or suggestions for this as you can.

Electric remote control Velux windows with rain sensors.
Cabled and fitted onto the lighting circuit with a fused switch and access point.

When I power up the velux via the fused switch (i.e. switch it on) the MCB trips.
Three possible scenarios -
cross wire/short
faulty switch
damaged cable

Cable tested fine before everything was sealed by the builders.

Changed the switch 4 times from 3 different manufacturers. Also extractor isolator at the side which is working fine. Disconnected the extractor and re-connected the velux into that switch. Tripped the MCB. Cant be the switch.

Bypassed the switch and powered up from the main CU and the velux works fine.

Re-connected switch. Tried to power up in reverse order - Switch then MCB then main box. As soon as the main box is powerd up the MCB trips.

I am absolutely baffled. The only scenario that works is the direct connecting of the velux to the lighting circuit.

Any ideas?

 

[tim west]

have you consulted the connection diagram for it?

 

[Spark123]

Does it have a fairly big transformer in it?

 

[securespark]

What were the builder's test readings?

Maybe the cable is down after being damaged. Test it again for IR.

 

[Chri5]

What fuse has the spur got in it ?

Would the lighting circuit be a 6a circuit and the Velux added on means the load for the circuit is exceeded ?

Why does the MCB trip when if the item was duff (which seems unlikely) the fuse in the spur should pop 1st?

Are you sure you haven't got the load / supply wiring twisted up ?

 

[RF Lighting]

Have you nicked one of the supply cables in the spur box?

This could cause a short when the spur is fitted to the box, and it might not show up it the spur is not fitted.

 

[oharaf]

Whats wrong with curtains!!!!