TNCS High ZE at origin

[AdamGiguere]

Why won't the DNO guarentee disconnection times...

Could you clarify the reasoning behind doing this in more detail for dummies () please first I'm coming across this...

 

[Wilko]

Why won't the DNO guarentee disconnection times...

Could you clarify the reasoning behind doing this in more detail for dummies () please first I'm coming across this...

Hi Adam - you are responsible for the disconnection times in the installation.

 

[AdamGiguere]

Ok but if my EFLI is within the parameters then I should achieve my disconnection times regardless whether they are on switch fuses right?

They are on double pole isolators via their main switch on the DB....

Slightly confused in this regard

 

[SparkyChick]

The service head fuse isn't there to protect your equipment, it's only there to protect the suppliers equipment and cabling.

So your protection only starts at the consumer unit. If that's 10m away from the metering location, that's 10m of effectively unprotected cable and you have no guarantee that a fault on that cable will be disconnected quickly under fault conditions.

Hence, runs longer than 3m require the presence of a suitably rated protective device... to protect the distribution cables that supply power to the remote consumer units.

 

[AdamGiguere]

Thanks Spark Chic!!!

Understood, what switch fuse would you advise to use that would be small enough to fit in this cupboard? as I do believe the tails at right end are also feeding cables with a run longer than 3 meters so I'd really need all 3 runs connected to individuals 60amp switch fuses

 

[SparkyChick]

Within the meter cupboard, these are suitable:-

80A Double Pole Fused Switch | ElectricalDirect - https://www.electricaldirect.co.uk/product/80a-double-pole-fused-switch-324279

In the house, you'd really need one in a metal enclosure to comply with the switch gear should be in non-flammable enclosure regulations. These are generally bigger. But from the looks of the meter cupboard, it's a bit tight already. If you could re-route the cables, then providing you can get the switch fuse within 3m, you could put them elsewhere and supply them with suitably rated meter tails.

 

[AdamGiguere]

Also why is it 3m's what if the board was 2m's away....

 

[SparkyChick]

The 3m limit is widely regarded as the maximum acceptable distance and it's the answer most people get when they enquire with the various distribution network operators.

You won't find it in BS 7671 because it's part of the DNO network design guides.

The best thing to do is measure up and have a chat with the DNO. My DNO is Western Power Distribution, their limit is 3m, but they don't seem to police it as I've worked on several properties which breach it. One place I've been to built in 2007 has tails over 10m long and no switch fuse, another has a 16mm T+E running about 25m.

 

[AdamGiguere]

Right i see!

The DNO have on this property don't seem to be policing this here.

They've not advised this work should be done.

If BS7671 doesn't stipulate and DNO is just a guide is this absolutely necessary?

Great info by the way - this forum is just what's needed!

 

[AdamGiguere]

Within the meter cupboard, these are suitable:-

80A Double Pole Fused Switch | ElectricalDirect - https://www.electricaldirect.co.uk/product/80a-double-pole-fused-switch-324279

In the house, you'd really need one in a metal enclosure to comply with the switch gear should be in non-flammable enclosure regulations. These are generally bigger. But from the looks of the meter cupboard, it's a bit tight already. If you could re-route the cables, then providing you can get the switch fuse within 3m, you could put them elsewhere and supply them with suitably rated meter tails.

Something like this would work if it combined the 3 sets of cables that are in there.

Could I not put a small 3 way board in there, as rerouting the cable would not be ideal, tenant wouldn't want to spend the money for the time it would take etc...

 

[SparkyChick]

Is it absolutely necessary?

Well, if you want to ensure the safety of the users of the installation and the property that contains it, then the answer should be yes. The DNOs don't police it because their responsibility ends where the tails leave the service head to go to the meter. From there up to and including the meter, the responsibility sits with the energy supplier and after that, it's the customers responsibility. The DNO can only advise.

Potentially yes, you could install a small board, but make sure you use fuse carriers, not MCBs or similar as if you don't you could have problems with discrimination under fault conditions.

 

[westward10]

Have you actually installed these cables or are they existing.

 

[SparkyChick]

Have you actually installed these cables or are they existing.

That's a very good point. I haven't addressed the issue at any of the properties I've encountered it. I have advised the customer, but as the installation has been that way for a long time, no one has taken up the advice.

 

[AdamGiguere]

The only cable I've installed is the 16mm T&E, everything else is existing...

 

[AdamGiguere]

Is it absolutely necessary?

Well, if you want to ensure the safety of the users of the installation and the property that contains it, then the answer should be yes. The DNOs don't police it because their responsibility ends where the tails leave the service head to go to the meter. From there up to and including the meter, the responsibility sits with the energy supplier and after that, it's the customers responsibility. The DNO can only advise.

Potentially yes, you could install a small board, but make sure you use fuse carriers, not MCBs or similar as if you don't you could have problems with discrimination under fault conditions.

Thanks, what fuse carriers fit in the new wylex metal boards?

 

[westward10]

The only cable I've installed is the 16mm T&E, everything else is existing...

To me that is very worrying that you are carrying out electrical works on other people's properties.

 

[AdamGiguere]

There has been two previous electricians as well who have not picked this up.

I'll be upgrading to a switch fuse as advised, but sparkychic what's the deal with existing then surely it all needs upgrading to switched fuses regardless of it being existing....

Westward I'm learning everyday.

 

[AdamGiguere]

So going forward such as today, I'm running 6mm swa from meter intake to outside garage.

I'll run the swa into a metal consumer unit in the garage.

At the meter I will upgrade to a double pole Henley block and have the swa on a 60amp switch fuse for protection of the 15m run.

I'll update the tails from the Henley block to existing fuseboard which is just next to the meter.

 

[AdamGiguere]

Quick Q - if feasible can an 50amp MCB be used off a fuseboard next to meter as opposed to a switch fused device.

If the total load will not exceed 40 amps...

 

[SparkyChick]

As previously stated, using an MCB can lead to discrimination issues under fault conditions.