Electrics upgrade

[xr4x4]

Currently waiting for quotes back, but just wanted to get some more opinions from you guys, as some of the suggestions I didn't think would work.

I'll explain,

We live on an old farm in a converted barn. Electric comes into us via overhead on a TT, to a fairly new cutout, meter, then a Wylex 63a 30ma stand alone rcd, then into a merlin gerlin isobar 4 evo, converted to single phase.

Out of this cu runs our curcuits, sockets, lights etc.

There are also 3 2 core swa cables running out to different locations around the farm, barn, stables and pool etc. these locations all have have separate cu with rcds.

Trouble is, ramdom tripping. If there's any problem anywhere else, the main 30ma rcd trips and cuts the power to the whole system, including us, in darkness!

Now, one of his ideas was to split the tails with henleys before the wylex, leave our install alone with the 30ma rcd, then run some new tails out the wall, directly into a outside box, into a 100ma td rcd, then into a cu with mcbs only, out of these 3 mcbs will then run back through the wall with t+e into separate boxes, then take the swa's out of the merlin cu into these separate boxes?

Therefore the swa are protected by the mcbs and the remote cu's will beable to do there job without affecting us and if there is a problem it can be accessed remotely.

What do you think?

Sorry for the long post.

 

[ericmark]

There are two main criteria with RCD's time and leakage current.
The leakage current can't remember is 300ma or 500ma think 300ma for farms as max. I can't remember if there is any max time.

The problem with a TT supply is both lives have to be switched both line and neutral and many of the domestic systems only switch line.

There are also rules on distribution units and consumer units the latter being a type tested distribution unit.

Earthing on farms does to my mind go OTT with huge earth mats on things like cow sheds.

So although your idea seems sound it would be very easy to fall foul of some regulations.

So if your asking general principles then yes all is OK with your idea. But if you want to ensure it complies with all regulations then really you do need some one on site to check as it is so easy to miss something when farms are involved.

The question on my inspection and testing involved a farm and I nearly just gave up and walked out as I had not studied the special regs on farms. Anyway I must have got it right as I passed but a lot was luck.

Basic with RCD is times 3. So 30ma fed by 100ma and 100ma fed by 300ma one would also increase times as well. I have worked on building sites where this has all been done. However when a daft worker knocked in a nail for his coat it still took all RCD's out. Four of them. Theory says it should not happen but in practice it does.

RCD's do not come in just 30ma @ 40ms but also there are versions which are far more resistance to tripping read this on X-Pole there are even auto resetting RCD's

Untitled

• ericmark
• 6 Dec 2008
• 34

but the price is something else last time I looked £350 each.

So if it were me I would do as you say but that does not mean it would comply with regulations and law. Part P is a big problem and I don't really know if it would cause a problem or not.

 

[xr4x4]

Ok thanks for that.

Is not really used for as a farm anymore, have got a few horses but they are kept in fields away from the area. It's just the best way to discribe what it looks like..!

I'd like to remove the 100ma rcd altogether, but being a TT, I'm guessing I need it?

 

[RF Lighting]

Earth mats on farms are not OTT, they are an essential part of the installation!

I don't like the idea of auto resetting RCDs one little bit. What if someone's still attatched to the circuit when they re-energise? Also it's going to leave intermittent faults un-noticed until they become permanant faults.

Farms require a maiximum RCD value of 300mA, but this is for fire prevention rather than prevention of electric shock, so there is no issue with the OPs proposal of a 100mA TD RCD to supply the submain circuits.

Also the use of a TD RCD as the OP proposes will prevent any problems happening as eric mentioned about a single fault taking out multiple RCDs

The plan sounds good, but I think you need an electrician on site to look it all over and check that all regulations for sepcial locations etc. are adhered to, as there is far too many variables with an installation like that to properly avise over the internet.

 

[RF Lighting]

Ok thanks for that.

Is not really used for as a farm anymore, have got a few horses but they are kept in fields away from the area. It's just the best way to discribe what it looks like..!

I'd like to remove the 100ma rcd altogether, but being a TT, I'm guessing I need it?

Yes, unfortunately the high impedence of an earth derived from an electrode means that RCD protection of all circuits is a must.

 

[xr4x4]

Ok cool, this will be carried out by an electrician but as I'm in the building trade, I like to know what's what and what people in the trade suggest is not always ideal for your situation...hence asking here.

Is there any other way you guys can think of doing it?

Again I'm guessing because it's TT, we have to have a rcd this end protecting the swa curcuits.

Trouble is this way, nuisance triping could still occur and due to the distance of some of the other buildings/curcuits, lots of walking backwards and forwards and phone calls are needed to identify the problem.

Due to the fact that all local curcuits will be protected by a 30ma rcd, what's the max rcd I can away with at supply end?

 

[Spark123]

A 100mA type S RCD ought to discriminate with a normal 30mA RCD.

The only other bit I don't like is having the whole of a house on one 30mA RCD. Maybe worth looking at splitting onto two RCDs?

 

[bernardgreen]

Putting the regulations aside for the moment. ( which means this idea may not be possible in reality )

Divide the incomer after the main fuse into separate supplies each with it own set of protection, MCB and earth leakage protection.

For the house a standard set of RCDs and MCBs

Protect the SWA sub mains from overcurrent with suitable MCB, detect damage and resultant leaks to the steel armour by a voltage operated RCB with its coil connected between the steel armour and the ground rod at the cut out. This will prevent the steel armour getting above 50 volts relative to ground so still "safe" to touch. Provided the steel armour is not connectted to ground in any way other than via the coil then the RCB will be tripped on any insualtion fault in the cable without there being any high fault currents through parallel paths to ground. But even if there were parallel paths and high fault current the limit would still be 50 volts. Most important this RCB will not be tripped by an earth fault in the outbuilding, it will only trip on damage to the SWA cable. So faults in the outbuilding can be cleared and its local RCB reset without having to go to the house.

At the out buildings have local RCBs protecting the local circuits.

 

[xr4x4]

A 100mA type S RCD ought to discriminate with a normal 30mA RCD.

The only other bit I don't like is having the whole of a house on one 30mA RCD. Maybe worth looking at splitting onto two RCDs?

Ought to? Trouble is, the owner might be a little peed off if they pay out all this money to have the system changed and still have to walk miles (not quite) to reset, maybe more than once in one sitting.

As regards to the one RCD, as the electrican put it, he can leave that install well alone, as electrics on old farms and barns can be bodged over time.

His quote affects this, as its twice the price to upgrade our CU as well.

 

[RF Lighting]

Putting the regulations aside for the moment. ( which means this idea may not be possible in reality )

Divide the incomer after the main fuse into separate supplies each with it own set of protection, MCB and earth leakage protection.

For the house a standard set of RCDs and MCBs

Protect the SWA sub mains from overcurrent with suitable MCB, detect damage and resultant leaks to the steel armour by a voltage operated RCB with its coil connected between the steel armour and the ground rod at the cut out. This will prevent the steel armour getting above 50 volts relative to ground so still "safe" to touch. Provided the steel armour is not connectted to ground in any way other than via the coil then the RCB will be tripped on any insualtion fault in the cable without there being any high fault currents through parallel paths to ground. But even if there were parallel paths and high fault current the limit would still be 50 volts. Most important this RCB will not be tripped by an earth fault in the outbuilding, it will only trip on damage to the SWA cable. So faults in the outbuilding can be cleared and its local RCB reset without having to go to the house.

At the out buildings have local RCBs protecting the local circuits.

Absoloutely crackers.

 

[xr4x4]

Regards as to this external box that is proposed, I'm guessing a ip 65 rated cu within a ip55 external metal box is ok?

Also I was thinking that he was going to have a separate 100ma rcd next to the cu, but I'm guessing it can go inside the cu?

So the tails will go through the wall directly into the cu, then into the rcd, then the mcbs. This way there won't be any external cables.

 

[Spark123]

A bog standard 100mA RCD will not, in most instances, discriminate with a bog standard 30mA device which is why a type S 100mA RCD upstream is a better design as it will allow a small amount of time for a 30mA device downstream to operate before it starts to operate itself.
Normal 100mA device = definately peeved off.
Type S 100mA device = a reduced chance of being peeved off.

 

[RF Lighting]

Upgrade to PME = everyones happy

 

[ban-all-sheds]

Yes, unfortunately the high impedence of an earth derived from an electrode means that RCD protection of all circuits is a must.

Even SWA supplying CUs which have RCD incomers?

 

[JohnW2]

Yes, unfortunately the high impedence of an earth derived from an electrode means that RCD protection of all circuits is a must.

Even SWA supplying CUs which have RCD incomers?

What else would/could provide fault protection for the SWA itself?

Kind Regards, John.