The importance of polarity

[Sparkgap]

Talking with one of the guys in the office about a mysterious boiler fault (runs, ignites, locks out) reminded me of an issue in a new boiler room some years ago. Three new gas boilers all installed at the same time, due to handover to client but one boiler kept locking out. It would go through the startup sequence, establish a flame then lock out. Each boiler had a mains inlet socket at the back with a plug and flying lead from the control panel. Looking at the backs of the boilers I noticed the offending one had the manufacturer's socket label (the one which tells you which connection is Live/Neutral/Earth) upside down. Sure enough the sparky/trained monkey who wired up the plug had swapped the wires over to conform to the label. Changed them back and boiler works perfectly. It would appear that flame detection on that boiler was referenced to mains earth/neutral and this threw it out.

 

[w0z]

No RCD?
If the boiler was connected between live and earth I'd have expected the RCD to trip.
Strange that it locked out though, perhaps the earth wasn't quite right...

 

[Sparkgap]

No RCD?
If the boiler was connected between live and earth I'd have expected the RCD to trip.
Strange that it locked out though, perhaps the earth wasn't quite right...

1) Direct feed from the control panel with its own MCBs. No RCD.
2) The issue was L/N swapped, not L/E.
I'm assuming that the FFD circuit referenced to neutral and with the L/N reversed was not getting the correct signal hence locking out despite the flame established.
I did have a sparky wire up a control panel I designed EXACTLY as shown on my design tender drawing, not as shown on the as-manufactured drawing inside! Luckily nothing went bang (think 240V into LV circuits!) and all worked once the wires were put in the correct terminals.