Incorrectly wired FCU?

[BucksDan]

Hello all

1st post...

I have recently moved in to a house with electric underfloor heating. House was built in 1985 but UFH installed in 2013 from what I can gather.

I noticed that the FCU faceplate for the UFH was cracked and was also getting warmer than I would like when in use. Therefore, I bought a MK FCU to swap with the existing (unbranded) FCU. Fairly simple swap of nice new brown and blue T&E. However, upon inspection, the neutral from the load side was wired into the supply side and vice versa. Line was wired correctly... I swapped it over and wired it correctly but I'm intrigued why the incorrect wiring still worked and what the possible implications could have been? As you can tell I'm no electrician.

Thanks for any insight!

Dan

 

[JohnW2]

However, upon inspection, the neutral from the load side was wired into the supply side and vice versa. Line was wired correctly... I swapped it over and wired it correctly but I'm intrigued why the incorrect wiring still worked and what the possible implications could have been?

Welcome to the forum!

You don't say whether it was a switched or unswitched FCU. If unswitched, it would make absolutely no difference - the two neutral terminals are simply connected together, so it would make no difference which way round it was connected. Even if it were a switched one, it would not really make any difference - the two neutral terminals are connected when the switch is 'on' and disconnected when it is 'off'. Even if there were a neon indicator in a switched FCU, even that would still work normally with 'reversed wiring'.

So, although 'wrong'/untidy, there were really no 'implications'.

Kind Regards, John

 

[BucksDan]

Thanks John. It is a switched FCU.

I appreciate you clarifying that for me.

Dan

 

[JohnW2]

Thanks John. It is a switched FCU. I appreciate you clarifying that for me.

You're welcome. Although I started off by saying that I didn't know whether it was switched or unswitched, as I went on to say, it doesn't really matter - it would work 'as expected' (and with no problems) with 'reversed' neutral connections whether it was switched or unswitched!

Kind Regards, John

 

[PBC_1966]

Swapping the live side won't make any difference to functionality or safety either, it just means that the fuse remains live when the switch is off.

For a switched FCU with a neon indicator though, it might result in some "strange" effects: The neon would certainly be on when the switch is on, but it may also glow more dimly when the switch is off. But if you swap both L & N supply/load on a neon FCU, then the neon will be on all the time, regardless of switch position.

The one thing you must not do is use the "L" side of the FCU for your neutral and the "N" side for live, otherwise the fuse would end up in the wrong side of the circuit.

 

[JohnW2]

The one thing you must not do is use the "L" side of the FCU for your neutral and the "N" side for live, otherwise the fuse would end up in the wrong side of the circuit.

Indeed - and nor, of course, connect L and N the wrong way around on the load size, since that would result in the wrong polarity being supplied to the connected equipment (which might not always matter, but quite often would).

Kind Regards, John