Wiring for electronics lab!

[scottstephen111]

Hi all.

I stay in a 3 bed flat and I'm currently converting 1 room as a workroom for electronic circuit testing.

I will require 6 x double sockets and a feed for a couple of flourescent lights.

Sockets..

The idea i had was to take a feed from a 30A breaker in the main CU (6mm T&E) to a mini CU immediately next to it with a 40A RCD then through to the workroom (6mm T&E) to a 45A double pole isolation switch then to the sockets as a mini ring with 2.5mm T&E. (Ring connected directly to the 45A switch)

Before anyone asks I WILL get it certified when ive wired it up. I have, however been advised that this is unsafe due to the fact that their is no isolation possible for neutral?? - I'm a bit stumped here!!... Am I missing something? - Do you think a double pole isolation switch between the CU and RCD is the answer or a dedicated CU in the workroom?

The reason I'm doing it this way is that the flat has NO RCD protection at present in the main CU but I wanted to add it to the lab..

Thanks

Scott

 

[Paul_C]

Hello,

I have, however been advised that this is unsafe due to the fact that their is no isolation possible for neutral??

I don't see how anyone comes to that conclusion. If you are using a double-pole isolation switch to cut off all power to the room, then it will open both sides of the circuit. The RCD will be a double-pole device too and will open both conductors when tripped/switched off. There would certainly be no problem with isolation with what you propose.

A couple of other points though: Do you really need to run a ring for six double sockets, which, I'm assuming, are likely to have very little load? Electronics benches are usually provided with plenty of sockets for convenience, not because of high load. A simple 20A radial circuit would quite likely suffice.

You mentioned lights but not how they fit into the overall plan. If you're planning on connecting them to the same feeder from the main board, do you really want them on the same RCD? A fault on something connected to a bench socket which trips out the RCD could then also plunge the room into darkness.

It might be better to run the feeder to a small 2- or 3-way board in the room, then you could use an RCBO for the sockets and feed the lights separately from an MCB to avoid that potential problem.

 

[scottstephen111]

Thankyou for that quick reply.

Yes, I'm puzzled too! - However, perhaps a small 2-way/3-way CU is the answer in the room, meaning that I could indeed run the lights too. (30A MCB in main CU being sufficient for feeding the room CU?) - Surely I wouldnt have to put an isolator beside the main CU too after the MCB?

Thanks, Scott

 

[1john]

You seem to be making a mountain out of a mole hill. a 4mm cable from the cu to a 30mA DP RCD and then 4mm cable along to each socket. the RCD provides DP isolation.

Do you really need a 30A circuit wouldn't a 16A/20A circuit be more than sufficient?

 

[bernardgreen]

The way my workshop bench is configured for mains supply is a number of 13 amp sockets supplied from a 13 amp plug. This provides a single point at which all equipment can be turned off.

You might need to think about a socket without RCD protection for equipment that normally has high earth currents from its mains filtering such as stepper motor drivers and variable frequency invertors.

Anti-static carpet is another thing to consider.

 

[scottstephen111]

You seem to be making a mountain out of a mole hill. a 4mm cable from the cu to a 30mA DP RCD and then 4mm cable along to each socket. the RCD provides DP isolation.

Do you really need a 30A circuit wouldn't a 16A/20A circuit be more than sufficient?

I agree BUT my uncle gave me a brand new reel of 2.5mm T&E that needs using .. Although a ring is overkill (agreed) it certainly cannot be unsafe?

Also, will the small CU in the workroom need an isolator switch too or will the RCD suffice? (Thinking about that isolation concern from that spark!)

 

[1john]

If you already have the 2.5 then why ont install a radial circuit?

 

[Paul_C]

30A MCB in main CU being sufficient for feeding the room CU?

I would think so, unless you're planning on the workshop loading being well over 7kW, or unless you want to make provision in case extra loads exceeding that sort of level might be added later. It seems unlikely for a small workroom fitted with half a dozen double sockets.

Surely I wouldnt have to put an isolator beside the main CU too after the MCB?

No need. The MCB will protect the feeder cable. You could either use the main switch in the workshop board as the isolator, or connect via a double-pole switch in a suitable location, e.g. right by the door. Personally, I'd put the isolation switch for the bench on just the circuit feeding the sockets, so that all such power can be turned off quickly but without also extinguishing the room lights.

I agree BUT my uncle gave me a brand new reel of 2.5mm T&E that needs using .. Although a ring is overkill (agreed) it certainly cannot be unsafe?

No it wouldn't. But if you run the sockets on a simple 20A radial circuit, then 2.5 sq. mm. would be sufficient anyway (assuming it's not going to be bundled with many other cables or installed in some other way which would severely derate it, which seems unlikely for what you are describing).

 

[scottstephen111]

Ok, lets get this right

30A MCB in main CU > Isolator switch in room > small CU in room with RCD > sockets

OR

30A MCB in main CU > small CU in room > isolator switch > sockets

Maybe this for a cu?

http://www.screwfix.com/p/mk-sentry-4-way-rcd-garage-consumer-unit/33248

Thanks for all the help, much appreciated!

 

[Paul_C]

How are you going to feed the room lights? Are they going to be powered via this addition, or fed from some other circuit entirely?

 

[1john]

20A mcb - 2.5mm to bench - stand alone 30mA RCD in enclosure (this gives you DP isolation too) - 2.5mm to sockets.

The cable between the CU and workbench cannot be concealed within the wall unless it is more than 50mm deep from the surface or withing an earthed metallic containment system.

This work is notifiable BTW.

 

[scottstephen111]

How are you going to feed the room lights? Are they going to be powered via this addition, or fed from some other circuit entirely?

Yes, - So a mini split load CU in the room would probably be the answer.... As for the cable running from the main CU, this is located on the other side of the room wall at the top so I was contemplating dropping cable through the wall cavity to the mini cu in the room then from the mini cu to the sockets.

so, final thoughts.... 30a breaker in ye olde main cu, small cu with double pole 45a rcd for sockets and 6a mcb for lights. Switch between rcd and sockets for emergency isolation? OK?

 

[mikhailfaradayski]

Before anyone asks I WILL get it certified when ive wired it up.

Really? Have you asked any electricians whether they will be happy to do this for you, or are you just guessing that somebody will commit fraud for you?

 

[scottstephen111]

Before anyone asks I WILL get it certified when ive wired it up.

Really? Have you asked any electricians whether they will be happy to do this for you, or are you just guessing that somebody will commit fraud for you?

Ooooh, thats a bit low... REMEMBER, this is all THEORY at the moment. I will draw a circuit diagram THEN take it to an electrician for assessment. I am a qualified electronics engineer and I am more than aware of compliance!

 

[EngStudent]

Ooooh, thats a bit low... REMEMBER, this is all THEORY at the moment. I will draw a circuit diagram THEN take it to an electrician for assessment. I am a qualified electronics engineer and I am more than aware of compliance!

I don't think an electrician is going to be that interested in a circuit diagram.

You're best off ringing a sparky up and having a chat with him. Tell him your plans & get him to come down to tell you what to do & what to put where. Once you've done all the work, had him inspect cable runs, terminations etc., he'll test it and sign it off.