couple of questions

[sharpe]

Hi, i have a coupe of question.

1. What sort of external earth block is used for an external MEB, used to connect the MEC and a sub-mains earth on a TNS system?

https://www.tlc-direct.co.uk/Products/TLEB8.html

Is this what i am looking for or am i barking up the wrong tree. Does it need to be fixed inside an enclosure?

2. Does a switch fuse always have to be rated lower than the main cutout? Or can it be equivalent to the main cutout?

For example on a 60a main cut out, does the switchfuse used to supply the sub-mains have to be 60a or less?

 

[PrenticeBoyofDerry]

1) When you say external do mean within an outside meter cabinet.
the block linked will act as a connection of the main earths and earth conductors. There are ones with less terminals for less connections. Use both top and bottom terminal for only one connected cable, so cable is nipped up by two screws.
If the joint is made outside the cabinet and open to the environment then it should be enclosed in a suitable IP rated enclosure.
2) Ideally discrimination should be in place and the fuse protecting downstream items be lesser rated than the device upstream. Then you need to consider the total load that the main fuse is having to protect, as there would normally be other loads combining a higher overall demand.
Why do you require a second 60A device downstream of the main fuse?
If you intend to overload the main fuse, it maybe very wise to consider updating existing main fuse size, tails and earthing conductors. That would require speaking to your provider.

 

[ericmark]

What do you mean by "a 60a main cut out"? This could be an isolator or a moulded breaker I have not seen moulded breakers used in a house with the exception of some very old ELCB-v devices which had a overload built it which are now banned anyway. As to an isolator or RCD this should be larger or equal to the load, a 63A RCD will either have a DNO fuse smaller than 63A or MCB's smaller than 63A either way it should not be able under normal circumstances from reaching the maximum rating.

 

[sharpe]

Sorry i wasn't more clear.

1. When i say external i meant outside the consumer unit, not within an outside meter cabinet. The reason for this is to split the tails for the new sub-mains, rather than run the submarines from the consumer unit.

2. The reason for the switch fuse is to protect the sub-mains cable as it will be over 3m long. I have spoken to the DNO they said there was no need to upgrade the main cutout because it would be able to cope with the new sub-mains, sounds dubious as its a 60a cutout but they said to crack on.

Perhaps if i explain further i will make more sense. The system is a Domestic TNS supply, and it has a 60a main cutout, which then feeds a 100a DP isolator fitted by the DNO. I want to split the tails, to run a sub-mains to the garage. The garage load will be a 32a ring, and 6a radial both RCD protected.

So the system will be

60a cutout
100a dp isolator
henley blocks
tails to consumer unit
tails to switch fuse
sub-mains to garage
garage consumer unit (40a 30ma RCCB)

What size switch fuse do i need? Ideally i would prefer a 40a switches, but the smallest i seem to be able to find is 60a.

 

[ccam108]

So the system will be

60a cutout
100a dp isolator
henley blocks
tails to consumer unit
tails to switch fuse
sub-mains to garage
garage consumer unit (40a 30ma RCCB)

I think there should probably be a meter in there somewhere

 

[ban-all-sheds]

Hi, i have a coupe of question.

Why do you have them?

What reason do you have for wanting to know the answers?

 

[EFLImpudence]

I think there should probably be a meter in there somewhere

What? Do you know how much it costs if you have a meter?

 

[ban-all-sheds]

More than it costs until you get caught not having one?

 

[JohnW2]

I think there should probably be a meter in there somewhere

Indeed. I presume it was just an omission in what the OP typed! There's no way that the DNO would have fitted an isolator upstream of the meter (or in the absence of a meter!).

Kind Regards, John

 

[AndyPRK]

1. It's fine to have a metal earth block INSIDE a house, without an enclosure.

2. I can understand why the DNO wouldn't want to upgrade their fuse. 60A and 60A submain is fine.

 

[securespark]

Apart from a lack of discrimination.

 

[JohnW2]

Apart from a lack of discrimination.

That doesn't seem to worry them. When there is a post-meter switch fuse because of long tails, as required by DNOs, it's quite common for it to have the same rating as the DNO's cutout fuse.

In any event, how low do you think that the rating of a sub-main fuse would have to be to give a high probability of discriminating wrt., say, an 80A cutout fuse?

Kind Regards, John