Sparks from Heating Controls Spur Box. Help Appreciated.

8 Mar 2016
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United Kingdom
I noticed that my CH HW programmer unit (Honeywell ST9400C) was not showing that it had any power to it. When I checked the spur box for the heating system it had the red neon light illuminated ok but there was definitely no life at the programmer unit. I switched the power off at the spur and the neon light went off but when I switched it back on it immediately made a loud bang and flew a good spark out of the switch. (Gave me a good fright!). I checked the main fuse board and the heating circuit had tripped but all else was ok. When I checked the actual switch that had gone bang I could find nothing unusual inside it (no burn marks or bare wires) nor could I find anything obvious inside the original Danfoss connections box next to the spur box. The Honeywell unit has been on for a few years now (replaced a previous Danfoss one) and the spur switch was from the original Danfoss installation at least 10/15 years ago. I have checked the serial number on the Honeywell unit (1006) so it isn't from the faulty batch I read about on the forum. Does anyone have any idea as to whether the dead programmer could have been at fault for the bang from the spur unit switch? Any opinions from someone with experience of this sort of thing would be gratefully received.
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Unlikely to be the switch or the programmer as the original fault.

But you don't fault find by asking questions on the web, you do tests using a meter!

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