Three Phase PME

[Big_Spark]

Steve, I am pleased you have stated the type of installation. Many old churches do indeed have TT supplies regardless of the local supply environment. I do not have a clue why, but the REC's seem to treat churches differently from normal installations, more in line with those in caravan parks, construction sites and special installations.

A contractor I sub work from does many churches every year as they take work from a specialist contractors in thei field in the East of England, and all the churches tend to be TT supplies with their own Earth provision.

I think the important aspect of this is your mention of RCD's, so viatl in this type of installation regardless od supply type.

I have no desire to teach you how to suck eggs, but I would suggest that you look at only those contractors specialised in working in this environment, not simply any contractor, the Chirch Commissioners may be helpful, but I would suggest you start with the local Bishop's Office.

Many contractors will not understand the nature of a Church environment, and you could end up paying for what you don't need or require.

As a heads up, a Builders in the east that specialise in Churches is Bakers of Danbury (Essex), they may be able to help, and an Electrical Contractor who knows his stuff is Justin Vickery of Vickery Electrical 3a, Perry Way, Witham, Essex CM8 3SX

Tel: 01376 519933

Jud's Mobile is 07710233142

If you do phone, then phone Jud on his Mobile or ask for him at the office, don't speak to his father..if you do , you'll know why I said not to

 

[stevesey]

Thanks FWL

I wonder if TT is because of increased risk of lightening stike. Not an issues for us as it's a small gospell hall that's no higher than the adjcent semis.

I'll try the building chaps at local CofE to see if they have any contractor reccomendations (perhaps the Diocese secretary as well).

One further question - all sockets are protected by 30ma RCDs in the CUs (lighting circuits are not). Is this sufficent for the regs? From what I can see 100ma RCDs (on each phase) are "preferred", but not mandatory (although I think we should head that way in the longer term - but at the moment I'm looking for immediate inspection issues).

Cheers

 

[plugwash]

on a TT system unless you can install an insanely good electrode system (and i really mean insanely good) you NEED to have everything on some form of rcd as overcurrent breakers will NOT disconnect earth faults in any reasonable time.

 

[mapj1]

That should read 'EVER' - consider a 10 amp earth current, quite respectable for a typical 1m earth rod at 240v will make the ground sizzle when it rains, give a painful step voltage for anyone in bare feet nearby, may lead to surprisingly large bills, but will never EVER blow the 100A company fuse
Seriously a 100mA or 300mA RCD at the incomer, ideally time delay to allow discrimination with more sensitive ones down-stream should be considered the minimum urgent upgrade if not converting to PME.
(and I'll add that I've never seen PME in a church either - my interest is bell ringing, rather than wiring them, but on ringing tours doing a few churches a day you get to see some right unholy electrics in churches - I'd agree they are in a class of their own - typical arrangements for tower and belfry lighting seem to be frozen in time about 1927 - I suspect in many cases they simply cannot guarantee the indoor/outdoor zone protection and isolation from external earths coming in from outside or through the floor requirement to be truly safe with PME.. ) (and metal bell-frames & sometimes structural iron work are often linked to the lightening conductor, but not always - no obvious pattern)
The only thing allowed up-stream of any RCD but after company meters could be a plastic box with a plastic handled isolator switch inside it. Anything requiring an earth has to be after the RCD.

An ordinary loop tester will allow an estimate of the earthing impedance. Realistically, as it may be hanging on by a thread and still measure OK - if its not visibly inspectable, perhaps a new rod should be added, and installed in the modern way in any case (inspection pit, proper cable of known diameter etc.)

 

[stevesey]

I think new earth rod is a given. And a new bond to the incoming water - pain here is that water enters the building in the opposite corner to the electric and under floor access is limited - still such is life.

Agree that 100ma time delay RCDs are very desireable - these may get combined with the inspection work or done very shortly afterwards. This will mean re-routing all the CU tails on the fuseboard, but that's no bad thing as when the cowboys converted from overhead to underground they mounted the cutout on stand-offs about 2in off the fuseboad partially obscuring a fuse box (since replaced/moved) and with some of the tails running behind it.

That raises another question can 4 pole RCDs be used for non-balanced installations or are they designed only to work with genuine three phase equipment? I thought they would compare the sum of the phases with the neutral current which would work for either, but then I came across one where it said the neutral needn't be connected - true if load is balanced there should be any neutral current but then what is the RCD measuring/comparing?

Reason for asking was that a nice TP&N distribution unit between the cutout and the individual CUs would really tidy up the installation.

 

[Big_Spark]

Stevesey, The Main switch of the installation can meet all the requirements of the Regs if you Install a 500mA Time Delayed RCD.

Churches are Special Locations, there are health and Safety concerns above and beyond simple electrical concerns. Should the lights fail in the dark due to a disconnected supply, there are many very real dangers that exist there for people unable to see to get out safely. Emergency lighting in these buildings is always minimal, usually only over the designated exit, which could mean a single 8w Luminaire in a building capable of holding up to 100 people.

However, installing an RCD or MCCB to meet the requirements of the installation may prove overly expensive, so it would likely be more economical to replace Isolators in DB's for smaller rated 500mA TD RCD's.

However, if it is possible it would be sensible to have the mains re-arranged so that all the power circuits are fed from common boards, and all lights seperate boards. The Power Boards being controlled by 30mA RCD's and the lighting boards 500mA RCD's, this ensures all aspects of Health and Safety and Electrical Safety are complied with.

 

[stevesey]

Emergency lighting is a little further down my list!

Yes replacing individual CU isolators with RCDs (time delay for the lights) would be easier - good point. The main hall lights are still on a cartridge CU and I was planning to move them to spare slots on the newer CUs. Putting all the lights on the one split load CU and fitting this with a TD RCCB would make sense.

Thanks for all the help

 

[stevesey]

A quick supplementary question.

How important is it to balance the load between phases within the building?

Under certain circumstances it looks pretty unbalanced at the moment say 30A/45A/60A for example - it's pretty difficult to think of a scheme that would cope with all the differnet uses of the building at various times of year and maintain sensible phase isolation. Whilst I can move a couple of things to improve the balance I assume I shouldn't worry to much at this scale (after all if one person is alone in the building using the cooker you could easily end up with 20/0/0 and there is nothing you can do about that).

 

[Big_Spark]

Steve, as much as possible you should balance the loads where practicable to do so. This will help to improve the efficasy of the supply and thus reduce the electric bills the installatiopn recieves.

In an installation with no dedicated 3 phase circuits then this effect and the saving will be minimal, however 3 phase supplies are supposed to be balanced to within 15%, the REC's actually have the powers to enforce it, they simply don't bother.

Obviously this is a special location and the loading on the circuits is bound to vary quite a bit depending on the use the church undergoes, so any balancing you do may be arbitrary to be honest.

In Churches I have worked on, that have had 3 phase supplies, we normally put all the lighting on one phase, including any external lighting, then all the other circuits split across the remaining two phases and try to get as even a spread across the phases as possible. This may mean that one phase only has a couple opf circuits on it, normally feeding heaters, then another will have say 6 circuits whilst the lighting will have the most, yet the loading will likely end up being noticably less on the lighting phase.

This is always a mine field at the best of times, In installations where there are 3 phase machinery, balancing of loads is paramount to ensure efficient and correct operation of machinery etc, but in this case it is not as critical, simply desireable.

 

[stevesey]

Thanks - I think I've managed to come up with a scheme that will suit (and can be achived without needing to crimp/extend cables at the distribution board). Major heating will be on 2 phases with lights and lesser heating on the other. Under heavy load this should achive something in the order of 60/60/50, under lighter loads the balance will get worse but overall the RECs neutral current should stay acceptably low.

Cheers

 

[Big_Spark]

No problem Steve, if you need any advice or simply want to awall to bounce an idea off just shout mate.

 

[plugwash]

In an installation with no dedicated 3 phase circuits then this effect and the saving will be minimal, however 3 phase supplies are supposed to be balanced to within 15%, the REC's actually have the powers to enforce it, they simply don't bother.

what exactly do they mean by balanced within 15% in the rules?

balanced within 15% when running at maximum possible load?
balanced within 15% all the time ? (sounds pretty much unacheviable given how loading can vary)
balanced within 15% based on average load of each phase? (to comply with this one would probablly involve taking long term measurements then balancing based on them)
or what?