Discuss MEM 3-phase board in the Commercial Electrical Advice area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

H

hightower

Morning all,

As the regulars know, I work in a school part-time (while sparking part-time), and I'm going to do a bit work for them - they need power dropping in for 11 PCs. No problem, 2 RFCs should do to share the leakage currents of the machines, bit of dado trunking round the room, RFC covers the additional earth return path needed, job's a good 'un.

What's more, there's a DB in the room too so the run is an absolute doddle. But where I need help is with finding a suitable RCBO for the board. It's a MEM 3-phase board, (see attachment), and the following ways are spare:

Y1 (but as you can see, the dual module of B1 sits in this space)
Y5 (but again, the dual module of R5 sits in this space)
R6
Y6 (protective device installed, but not in use)
B6

There-in lies my problem. If I'm to use these BS3871 devices I need four spare ways, and I only have three. So I've got two possible solutions:

1) Somebody suggests a single module RCBO that I can use in this board (opening the board up, there's not a lot of head space between the breakers and the enclosure, so really not sure this is a solution.

2) B5 is only feeding a solitary CCTV camera, it's wired in 2.5mm and on a 6A. I could drop this cable in with say, B4 - Alarm Spur in Corridor, or any number of 6A breakers feeding lights.

So I guess option 1 would be my preference, but does anybody see a problem with option 2 should option 1 not be feasible?

Also, a sub-question, just out of interest. Every lighting circuit from this board has 2.5mm down as the cable size - I'm guessing they've done this to account for voltage drop on longer runs. Question is, why have they wired the CCTV in 2.5mm too - it's literally 3m from the board.

View attachment 35354
 
Last edited by a moderator:
option 2. shift the CCTV over to another way.

Thanks Tel. I'm thinking a Type B breaker too would be fine instead of the Type C that is sitting spare. I know the PCs will have a higher startup current but there's only 6 per circuit and I can't see them knocking a Type B off.
 
They aren't listed in the current regs, so how are you going to correctly certify your work?


BS 3871 mcb's - https://talk.electriciansforums.co.uk/topic/18057-bs-3871-mcbs/
 
I've been told before not to put rcbos on sockets if they are designated for computer circuits only due to the possible high earth leakage
 
They aren't listed in the current regs, so how are you going to correctly certify your work?


BS 3871 mcb's - https://talk.electriciansforums.co.uk/topic/18057-bs-3871-mcbs/

The values for maximum Zs aren't listed in the current regulations, but that doesn't mean BS 7671 frowns upon their use. BS 7671 basically states that an overcurrent protective device or an RCD may be used for fault protection under ADS (which this BS 3871 would tick the box for) and that the characteristics of the protective devices and the circuit impedances shall fulfil the following requirement: Zs x Ia <= Uo

Zs = 0.7ohms for the sake of argument. My design tells me it would be far lower though.
Ia = 224A if I've read the table correctly

0.7 x 224 = 157 which is lower than our Uo of 230v. Looks compliant to me, unless I'm missing a regulation that specifically bans their use?

EDIT: Sorry, working off a slightly older regs. I know BYB introduces Cmin too, but even with this factor applied it will be compliant.
 
Cant you take another sub board from the main board?

I could probably take a sub board from this board, but we'd still have the same debate about reusing BS 3871s to protect the sub board circuit. I can't take another sub board from the main main board if that's what you mean.

It's not an issue to remove that 6A circuit and then I have 4 spare ways - which is what I need. In fact, the CCTV camera is supplies is dated and it'll be on a todo list somewhere anyway. When it does get replaced it will be with a POE IP camera, so moving it off temporarily isn't a major problem.

Was just hoping members could recommend a replacement breaker that fits, but I've done a fair bit of research today and it seems we're now up to MEM3 boards which offer no backwards compatibility.

Has anyone got any of these devices that I could buy off you?
 
We tests lots of schools and the ICT Suite normally has its own board, all rcbos. Each ring final doesn't generally exceed seven outlets so I would say your plans are sound. I wouldn't worry about using 3871 devices at all. Should easily find some Memshield 1 on the net, I see Lee has. They may say type C but they are probably still BS3871.
 
Bloody typical. I was adding two RFCs to the board, using an eBay special and a C32 that was already sitting in there spare. eBay special works a treat, everything tests fine and happy as with it.

No, my problem exists with the C32 that was sitting spare: neutral and earth flying leads were disconnected and taped up. Now if I was going to leave an RCBO in-situ but unused, I think I'd be leaving the fly leads attached to their respective busbars too. Which begs the question - why were they removed and taped up but the device was still in the board? I'm in two minds whether to use this, what's everyone's thoughts? I haven't even tested it. Would you connect the second RFC to this and run it through all the usual tests and be happy using it if everything checked out? I suppose it can't be any worse than trying a second hand unit off eBay.

Just racking my brains why the neutral and earth fly leads were disconnected and taped up. Probably overthinking it.
 
This is a strange coincidence because I tested a school at the last term break, Memshield 2 and the spare rcbo leads had been disconnected and taped up. Just connect it and test, nothing to lose.
 

Reply to MEM 3-phase board in the Commercial Electrical Advice area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock