110v supply issue?

[Steve46]

just a quick question I’ve searched online but want to be sure before deciding what to do.

I am putting in a supply for x5 110v commando sockets in a workshop.

The commando sockets each have a internal 16amp double pole MCB inside the socket.

I’ve wired from the DB a 32amp 10mm2 supply to a centre tapped transformer as per drawing.

From here I was unsure how to wire out to the x5 commandos.

So I ran a cable from the transformer in to a small 6 way DB and and fed the commando sockets from there the issue I have is that I’ve used x5 16amp single pole MCBs, then thought that they might need to be double pole MCB’s.

Would I need to change the small Db for a larger one and feed the commandos with x5 double pole MCBs?

thank you for your time and have a great day ??

 

[telectrix]

they should be d/p MCBs , IMO, as you have 2 "live" legs, each 55V relative to earth.

 

[Lucien Nunes]

If the transformer centre-tap is earthed then yes, all switching and protective devices must be double-pole, as both the other wires are lines and need protecting against faults to earth and both must be switched otherwise the appliance remains live.

 

[Mike Johnson]

The commando sockets each have a internal 16amp double pole MCB inside the socket.

Now that has confused me? why do you need more?

 

[Strima]

Why not run it as an RFC and try and get the legs the same length the balance the load?

 

[Steve46]

The sockets come with built in double pole mcbs, the problem I had is they are all fed from 1 transformer so I have 5 cables at the transformer that’s why I was planning to put a small db in next to the transformer?
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Cheers for the replies ??

 

[Mike Johnson]

Would one of these do: Legrand EN 60947-1 048-84 Modular Distribution Block 100A-40 Ui 500V ipk 20ka | eBay - https://www.ebay.co.uk/itm/Legrand-EN-60947-1-048-84-Modular-Distribution-Block-100A-40-Ui-500V-ipk-20ka-/302637580524
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You will probably need Bootlace ferrules and a crimping tool if you don't have them already, you can get the Legrand distribution box in the UK.

 

[davesparks]

What rating is the output of the transformer and does it have any output protection built in?
Have you considered a single radial circuit from the output of the transformer?

If not yes the DB will need DP MCB's. Most of the bigger brands of DB have the facility to fit distributed neutral and DP MCB's.
Personally I prefer the schneider Acti 9 range for this.
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Would one of these do: Legrand EN 60947-1 048-84 Modular Distribution Block 100A-40 Ui 500V ipk 20ka | eBay - https://www.ebay.co.uk/itm/Legrand-EN-60947-1-048-84-Modular-Distribution-Block-100A-40-Ui-500V-ipk-20ka-/302637580524
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You will probably need Bootlace ferrules and a crimping tool if you don't have them already, you can get the Legrand distribution box in the UK.

No, it won't do, the OP needs a DB with a neutral kit and DP MCB's.

 

[Mike Johnson]

He's already got double pole MCB's in the socket, why does he need additional double pole MCB's in a DB? I suppose it all accords how long and vulnerable the lead are.

 

[pc1966]

He's already got double pole MCB's in the socket, why does he need additional double pole MCB's in a DB? I suppose it all accords how long and vulnerable the lead are.

He said it is a workshop, so I guess the cable will be fixed and (hopefully) well protected physically, not like trailing leads on building sites that the folk just drive over!

 

[Ian1981]

He's already got double pole MCB's in the socket, why does he need additional double pole MCB's in a DB? I suppose it all accords how long and vulnerable the lead are.

Double pole mcb’s as effectively you have two line conductors each needing fault protection and is required by regulation 411.8.3

 

[pc1966]

Double pole mcb’s as effectively you have two line conductors each needing fault protection and is required by regulation 411.8.3

Having MCB at both ends is not terribly good from a discrimination point of view.

No information on the transformer rating but it might be a case of having some fuses at the transformer outlet/DB rate for full 110V load, and sufficient conductor cross-section to match, maybe a ring?

 

[Steve46]

The transformers rated output is 3.0kva. The commandos are wired in 2.5mm swa on tray. 5 radials from 15 to 35 metre runs

 

[pc1966]

3kVA at 110V is just over 27A so you could consider having a pair of 32A fuses on its output before splitting to the sockets, that ought to be OK for protecting the 2.5mm wire against a major short. The 32A breaker on (presumably?) the primary would be equivalent to 67A on the 110V side.

Based on the (admittedly probably not that accurate) Eaton coordination tables "TD01400002U – April 2013" you should get discrimination to around 0.7kA fault currents between 16A MCB and the common 32A BS88 style fuses. I suspect you won't get that sort of PFC with your cable runs anyway!

 

[pc1966]

Actually thinking about it, a 32A breaker feeding a 3kVA transformer sounds a lot more than is sensible, is it on the output?

If is is a DP on the output you already have you cable protection, but you really would need it to be a D-curve to have any hope of coordinating with the commando socket's 16A MCBs (assuming they are B-curve).

Also if that 32A is on the primary and you have the misfortune of a 55V-0V short it would look more like a 134A breaker on the secondary and that will not end well for 2.5mm cable!

 

[Steve46]

Cheers lads I’m going to email the consultants that issued the drawing in morning to ask what they specifically require I’ll let you know what they come back with

 

[pc1966]

Will be interesting to see what they have in mind. It is possible the 32A primary MCB was chosen to deal with switch on surges, but it may be better to use something like a 16A or 20A D-curve MCB instead.

Protecting the output wires to the sockets in the (hopefully unlikely) case of a short still seems to be missing but hopefully you will hear by tomorrow or Monday.

 

[davesparks]

He's already got double pole MCB's in the socket, why does he need additional double pole MCB's in a DB? I suppose it all accords how long and vulnerable the lead are.

For the same reason we supply socket circuits from an MCB when every plug has a fuse in it.
The circuit feeding the socket needs to be correctly protected for overcurrent and fault.

 

[pc1966]

I guess it also falls under 433.2.2 as the cables are longer than 3m and the output of the transformer could reasonably be taken as a "point where a change occurs".

 

[Mike Johnson]

Why don't building site transformers have MCB's on the fly leads to what they are supplying?