SUB MAIN QUESTIONS. ADVICE NEEDED

[Nelly72]

Hi all.

I have been asked to take over a job an electrician has started, and has moved abroad.

The house is a big mansion, with a 3PH supply, with 100A fuse on each Phase.

The electrician has ran 2 25mm 3core armours to another 2 DB’s in the mansion, and there has to be a DB fitted where the incoming supply is, bringing it to a total of 3 DBs.

In the plant room, there is also a 3PH heat pump.

However, my question is how would you all terminate the sub mains. Would you bring each phase into a Henley block, and branch off into a 80A switched fuse for the armour sub mains, and take a tail off the Henley blocks for each phase into a separate enclosure with a protective device for the heat pump, or would you fit a TPN board, and bring all armours into it + the heat pump supply?

If it’s a TPN board would you fit MCBs or MCCBs for the 25mm armours? Is there even

I’m a new build house basher, so this is abit more complex to what I’m used to, even though it’s a new build, so please go easy on me I am simply just wanting advice to keep me right.

 

[westward10]

Never seen a three phase CU and the pump is not likely to need an RCD.

 

[Rockingit]

Why not just put a 6 way Ryefield downstream of the DNO incomer? Gives you a box to gland into, choice of fuse sizes and a better disconnection time. 2 ways for the subs, 3 ways for the plant room (which I'm assuming will need a 3ph board in it regardless) plus a spare for the future.

 

[westward10]

Sounds like the client has already obtained the parts so other options are not likely to be favoured.

 

[westward10]

Just a CU with a three phase RCD to supply the three phase air pump and to distribute the single phase's to the other CU's.


Each subsequent CU to have a single phase RCD or as many as you want.

Mcbs are not likely to achieve selectivity. Using such devices for distribution is a poor design.

 

[Rockingit]

Sounds like the client has already obtained the parts so other options are not likely to be favoured.

Something about the blind leading the blind. At £90 a pop, plus all the other Henleys, Tails, time.... small MEM board would be cheaper!!

 

[plugsandsparks]

Came across a TP board with a RCD where one would normally find a 60947-3, thought that was odd. Then about a month later found another one, it must have been a thing 20-30 years ago. Think they were MEM boards but cannot be sure.

 

[Mike Johnson]

It's how I was instructed to do it by the French "Conusule" who are the regulating body in France, the incoming main is into a S type RCD at the meter this is EDF equipment you have no choice.

 

[DPG]

It's how I was instructed to do it by the French "Conusule" who are the regulating body in France, the incoming main is into a S type RCD at the meter this is EDF equipment you have no choice.

Luckily the OP is not in France.

 

[plugsandsparks]

Makes Sense, although these were in the UK and well downstream of the DNO head and pretty sure they were not S type but cannot be sure BUT there were no RCDs on the final ccts. It all looked strange to me

 

[plugsandsparks]

There is still one on a site i go to occassionally will try and get a photo.. as i would like to know if its was once a thing.....

 

[nicebutdim]

It's how I was instructed to do it by the French "Conusule" who are the regulating body in France, the incoming main is into a S type RCD at the meter this is EDF equipment you have no choice.

That would also be a TT supply?

 

[oscar21]

Mcbs are not likely to achieve selectivity. Using such devices for distribution is a poor design.

I had this argument with someone a while ago, he wanted to protect a sub main to a summer house with a 40A MCB in the consumer unit and I was saying that if the 6A lighting circuit has a problem it could well trip the 40A MCB before the 6A one, it just depends which is the quickest to go.

I was trying to explain that if you did a loop impedance test/IPF and it showed more than say 200A which it could well do, then if there was a dead short it could take out the 40A one instead, or maybe both but he was adamant thats what 6A MCBs are for and just couldn't understand it.

 

[pc1966]

I had this argument with someone a while ago, he wanted to protect a sub main to a summer house with a 40A MCB

Selectivity will be poor, but sometimes it comes down to how important it is. For an occasional summer house it is probably acceptable risk, but not for many circuits in a home where loss of stair lighting, etc, has other hazards.

Next time point them to the tables of selectivity as that will give figures for PFC/PSCC that are likely met, but in most cases you will find the end circuit exceed them so a real risk of supply loss.

That is why fuses or MCCB are the best choices, then the Ryefield or MCCB panel choice usually comes down to likelihood of a trip and need for non-skilled folks to reset.

 

[oscar21]

Are MCCB's less "sensitive" than an MCB then like fuses are, I'm no expert in commercial stuff.

 

[pc1966]

Are MCCB's less "sensitive" than an MCB then like fuses are, I'm no expert in commercial stuff.

Once you get pas the slow thermal trip side, many MCCB have a short-delay aspect (sometimes adjustable) of 20ms or so at modest overload currents, then once you hit really big faults in the kA range they go on an "instant" trip in the 10ms or less region to energy-limit the fault, just as MCB do.

It is this short-delay aspect that gives them decent selectivity with MCB that all go "instantly" as soon as you get past the thermal limit. That is one major advantage for applications like this where you are feeding MCB/RCBO boards.

The other aspect that help with MCCB is they often have adjustable settings, for the cheaper ones (and they are not cheap, this is a relative term) that is usually just the thermal trip point and you might get 80-100% adjustment on a given size, but that is enough so with a bit of choice you can design for any specific limit. For the fancy electronic ones you can dial in all sorts of trip points to get the sort of characteristics you need, and they can comes with RCCB aspects as well giving you a very fancy RCBO to practically any trip characteristic you want. At a price!

Fuses are old technology and cheap by comparison, but they naturally have a "delay" in how the wire heats up, and they offer exceptionally good energy-limiting. So if you really don't expect them to be blown/tripped, they are a very good alternative and will just sit there for decade after decade after decade, and still work if called upon.