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Electrical Question.

Mullardman

Moderately extreme...
I'm fitting a 'bulkhead light' on the side of the house. I'm powering it as a 'spur' from a power socket in the integral garage..via a Fused Connection Unit fitted with a 3A fuse. This because picking up a supply from anywhere on the lighting circuits would be very messy.

So it is 13A power socket via 2.5mm flat twin and earth to Fused Control Unit with 3A fuse, via 1.0mm flat twin and earth to 5A junction box and with 'live' taken via a wall mounted switch (through the wall in the dining room) to the bulkhead light ..on the outside wall.

All the usual safety.. earth sleeving.. red tags on the black/blue 'return' from switches etc. will be observed..

However.. what is foxing me is which terminal to use for what in the FCU?

All the pics of FCUs I've seen have clearly marked 'Mains' and 'Load' terminals... giving the impression that this distinction matters. The one I purchased has no internal labelling, but is well made and supplied by Screwfix.

After some puzzlement, I established that both Neutral terminals are 'commoned' via a brass 'bus'.
Both Live terminals as you'd expect.. sit either side of the fuse.

So... does it actually matter which Live terminal the supply and load are connected to? I can't see how it does.. Am I missing something?
 
I'm fitting a 'bulkhead light' on the side of the house. I'm powering it as a 'spur' from a power socket in the integral garage..via a Fused Connection Unit fitted with a 3A fuse. This because picking up a supply from anywhere on the lighting circuits would be very messy.

So it is 13A power socket via 2.5mm flat twin and earth to Fused Control Unit with 3A fuse, via 1.0mm flat twin and earth to 5A junction box and with 'live' taken via a wall mounted switch (through the wall in the dining room) to the bulkhead light ..on the outside wall.

All the usual safety.. earth sleeving.. red tags on the black/blue 'return' from switches etc. will be observed..

However.. what is foxing me is which terminal to use for what in the FCU?

All the pics of FCUs I've seen have clearly marked 'Mains' and 'Load' terminals... giving the impression that this distinction matters. The one I purchased has no internal labelling, but is well made and supplied by Screwfix.

After some puzzlement, I established that both Neutral terminals are 'commoned' via a brass 'bus'.
Both Live terminals as you'd expect.. sit either side of the fuse.

So... does it actually matter which Live terminal the supply and load are connected to? I can't see how it does.. Am I missing something?
Is it just a fuse that’s sittings between the “supply” and “load” terminals? If so then no, it doesn’t matter (fuses aren’t directional, not even audiophile types :D).

I don’t mean this to come across as condescending, but if you’re at all unsure of what you’re doing, find someone that does, you only need to make one mistake with mains electricity and it can be your last one.
 
I'm fitting a 'bulkhead light' on the side of the house. I'm powering it as a 'spur' from a power socket in the integral garage..via a Fused Connection Unit fitted with a 3A fuse. This because picking up a supply from anywhere on the lighting circuits would be very messy.

So it is 13A power socket via 2.5mm flat twin and earth to Fused Control Unit with 3A fuse, via 1.0mm flat twin and earth to 5A junction box and with 'live' taken via a wall mounted switch (through the wall in the dining room) to the bulkhead light ..on the outside wall.

All the usual safety.. earth sleeving.. red tags on the black/blue 'return' from switches etc. will be observed..

However.. what is foxing me is which terminal to use for what in the FCU?

All the pics of FCUs I've seen have clearly marked 'Mains' and 'Load' terminals... giving the impression that this distinction matters. The one I purchased has no internal labelling, but is well made and supplied by Screwfix.

After some puzzlement, I established that both Neutral terminals are 'commoned' via a brass 'bus'.
Both Live terminals as you'd expect.. sit either side of the fuse.

So... does it actually matter which Live terminal the supply and load are connected to? I can't see how it does.. Am I missing something?


sorry if you need to ask the question, then perhaps you should get someone in who understands what they are doing, and who is qualified to sign off the work.
 
I'm fitting a 'bulkhead light' on the side of the house. I'm powering it as a 'spur' from a power socket in the integral garage..via a Fused Connection Unit fitted with a 3A fuse. This because picking up a supply from anywhere on the lighting circuits would be very messy.

So it is 13A power socket via 2.5mm flat twin and earth to Fused Control Unit with 3A fuse, via 1.0mm flat twin and earth to 5A junction box and with 'live' taken via a wall mounted switch (through the wall in the dining room) to the bulkhead light ..on the outside wall.

All the usual safety.. earth sleeving.. red tags on the black/blue 'return' from switches etc. will be observed..

However.. what is foxing me is which terminal to use for what in the FCU?

All the pics of FCUs I've seen have clearly marked 'Mains' and 'Load' terminals... giving the impression that this distinction matters. The one I purchased has no internal labelling, but is well made and supplied by Screwfix.

After some puzzlement, I established that both Neutral terminals are 'commoned' via a brass 'bus'.
Both Live terminals as you'd expect.. sit either side of the fuse.

So... does it actually matter which Live terminal the supply and load are connected to? I can't see how it does.. Am I missing something?

If it's just a FCU with no switch then it doesn't really matter, if it has a switch the fuse should be 'off' when the switch is off. I suspect as it's not labelled that it has no switch and if so it does not matter.
 
If you are controlling an external light then it is a better option to have a DP switch in case the light develops a fault and trips off the RCD protecting the circuit you have spured from. If you have a neutral earth fault a single pole switch will not help. Why do you require a 5a junction box ?
 
It doesn't matter if there no switch. Is the circuit RCD protected though? If not you really should get an integral RCD FCU
 
sorry if you need to ask the question, then perhaps you should get someone in who understands what they are doing, and who is qualified to sign off the work.

I understand what I'm doing. I'm creating a fused spur off a power socket to power a bulkhead light. I asked the question to be extra sure. It's easy to stand back and be holier than thou. It's more challenging to think the job through and implement it.. taking advice where needed.
If I didn't understand what I was doing.. I'd run a bit of twin flex from an old BC light socket adapter...
 
I understand what I'm doing. I'm creating a fused spur off a power socket to power a bulkhead light. I asked the question to be extra sure. It's easy to stand back and be holier than thou. It's more challenging to think the job through and implement it.. taking advice where needed.

it’s also the best and safest way of learning mate, no one is born with this knowledge, everyone has to learn
 
It doesn't matter if there no switch. Is the circuit RCD protected though? If not you really should get an integral RCD FCU

The whole house installation is protected by a modern consumer unit with RCDs, installed around 2 years ago by a qualified electrician who also wired my new kitchen.

If you are controlling an external light then it is a better option to have a DP switch in case the light develops a fault and trips off the RCD protecting the circuit you have spured from. If you have a neutral earth fault a single pole switch will not help. Why do you require a 5a junction box ?

Thanks for the tip re the switch.

In order to be able to switch off the light from indoors rather than having to go out to the garage where the bulk of the wiring is.... I'm converting a 1 gang light switch indoors (other side of wall from garage) into a 2 gang switch and using gang 2 to control the outside light. (although once set up it should operate automatically most of the time)
The need for this switch is why I need a junction box.
 
If there’s a red neon indicator on the FCU it will always be illuminated if you get the load and line live connections mixed up.

Convention would be permanent live to top connection I would think.
 
I'd have thought that too but seemingly not. The one I bought was sold and pictured as below.. by Screwfix... but came without the 'Load' and 'In' markings.

ae235

I also note that the assembly screws which should be in the holes between live and neutral pairs are missing... :eek:....
But not on mine....
 
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The whole house installation is protected by a modern consumer unit with RCDs, installed around 2 years ago by a qualified electrician who also wired my new kitchen.

Good. It's important to determine because even wired fuses are still allowed, as long as everything is 'safe'. Don't buy cheap unbranded stuff if you can avoid it - the e.g. MK stuff is much better, there's some real crap out there.

 
Yep. I have used both MK and Crabtree stuff in places. However.. this is 'British General' which looked ok and is well enough made... just a bit puzzling.
 
Yep. I have used both MK and Crabtree stuff in places. However.. this is 'British General' which looked ok and is well enough made... just a bit puzzling.

I think Crabtree is now Indian owned, it's not the same company any longer for sure, but I can't say anything about the quality. There's also a new, cheaper MK range that's just come out that's also 'imported'
 
Yep. I have used both MK and Crabtree stuff in places. However.. this is 'British General' which looked ok and is well enough made... just a bit puzzling.

I'm not keen on BG stuff as often they are difficult to get the terminals to tighten up without damaging the heads (made from very low grade metal) and/or because the terminal bolts are more 'pointed' than they should be. Have to say I like MK stuff (even if it's not as good as it once was) or even LAP (Screwfix) seems OK.
 
It sounds like a duff one. It would have to be marked to comply with British Standards.

It is marked L, N, E etc and with BG.. 50/60 Hz 250/13 etc.. and BS 1363-4. It just doesn't indicate 'Load' and 'Supply'

Don't assume just because it's from Screwfix that it is any good, they sell some right crap.

Point taken.

I'm not keen on BG stuff as often they are difficult to get the terminals to tighten up without damaging the heads (made from very low grade metal) and/or because the terminal bolts are more 'pointed' than they should be. Have to say I like MK stuff (even if it's not as good as it once was) or even LAP (Screwfix) seems OK.

Thanks... I think the BG thing is just about serviceable and once in shouldn't need further attention. But.. It's a case of 'once bitten' and I won't buy any more of their stuff.

All the original sockets, switches etc in my house are 'Tenby' and although they now look dated and a bit naff.. none have failed in 44 years. Tenby seem to still be around in some form.
 


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