16mm tails

[TicklyT]

Our service head is fitted with a 100 Amp fuse. The 25mm tails from our meter are connected via henly blocks, and 25mm tails to the main CU. There are also 16mm tails from the henlys to a 60Amp Wylex switch fuse feeding a second CU.

It has been suggested that the existing 16mm tails feeding the switch fuse should be upgraded to 25mm to comply with current regulations. Is this correct, and will 25mm even fit into a Wylex switch fuse?

Because you like piccies.....

 

[Spark123]

I'm not seeing much of a problem, if overcurrent protection is provided by the switchfuse and fault current protection is provided by the cutout fuse (adiabatic equation) then afaik the 16mm² tails are OK.
I can't see the banjo on the SWA gland though??

 

[Adam_151]

Like spark123, I can't see much of a problem, if you did the calcs you could prove compliance to BS7671 (at least in all but very extreme cases), the only issue is if the DNO have any over-rideing conditions what what they will allow their fuse to protect (they can be funny sometimes, just think how possesive they can be of square of chipboard...)

EDIT: The jury is still out over whether BS7671 requires a single isolator for the whole install ahead of the henley blocks...

 

[TicklyT]

I can't see the banjo on the SWA gland though??

Good point.

I didn't install it, I just own it.

Just checked the SWA cable a bit more closely.... It's a 3 - core.

It's about 17.5mm Diameter. As it's 3-core, it looks like it must be a 6mm cable on the end of the 60 Amp fuse, rather than 10mm 2-core.

Does this open another can of worms?

I suppose the armour is earthed, but only via the switch fuse case .

The SWA run is about 4 Metres total, 2 Metres clipped to an external wall a good 2 Metres high, then back inside to the second CU, which has an RCD on the incoming, one 6A MCB for lighting and one 32A MCB for power. Maybe a 60 amp fuse in the switch fuse is a bit OTT.

Had a PIR done on it a couple of weeks ago, and the only comment then was marking the 16mm tails down as a '2'

 

[Harry The Hole]

I can't see the banjo on the SWA gland though??

Good point.

I didn't install it, I just own it.

Just checked the SWA cable a bit more closely.... It's a 3 - core.

It's about 17.5mm Diameter. As it's 3-core, it looks like it must be a 6mm cable on the end of the 60 Amp fuse, rather than 10mm 2-core.

Does this open another can of worms?

I suppose the armour is earthed, but only via the switch fuse case .

The SWA run is about 4 Metres total, 2 Metres clipped to an external wall a good 2 Metres high, then back inside to the second CU, which has an RCD on the incoming, one 6A MCB for lighting and one 32A MCB for power. Maybe a 60 amp fuse in the switch fuse is a bit OTT.

Had a PIR done on it a couple of weeks ago, and the only comment then was marking the 16mm tails down as a '2'

In theory you should be using 25mm tails all round. I was jumped on by an NICEIC Engineer over the same issue last year. He went into all sorts of earth fault mode.

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[Spark123]

Why 25mm²? Like adam says, the only reasonable explination I can see that would force you to go up to 25mm² for a 60A CU will be some sort of ruling by the DNO.

 

[Spark123]

I can't see the banjo on the SWA gland though??

Good point.

I didn't install it, I just own it.

Just checked the SWA cable a bit more closely.... It's a 3 - core.

It's about 17.5mm Diameter. As it's 3-core, it looks like it must be a 6mm cable on the end of the 60 Amp fuse, rather than 10mm 2-core.

Does this open another can of worms?

I suppose the armour is earthed, but only via the switch fuse case .

The SWA run is about 4 Metres total, 2 Metres clipped to an external wall a good 2 Metres high, then back inside to the second CU, which has an RCD on the incoming, one 6A MCB for lighting and one 32A MCB for power. Maybe a 60 amp fuse in the switch fuse is a bit OTT.

Had a PIR done on it a couple of weeks ago, and the only comment then was marking the 16mm tails down as a '2'

Maybe the banjo is on the inside of the CU? I'd have hoped your spark would have picked up if it was missing. Is it a 60A fuse under the cover? Just seems to me that 6mm² would be a bit on the small side for a 60A supply. Anyone got their charts to hand? Saying that the same rules apply, if the 60A fuse is providing fault protection (adiabatic) and the circuits are limited to 38A then it may still comply, the problem with this type of setup is someone adding load to the second CU.

 

[davy_owen_88]

6mm² SWA clipped direct is rated at 49A.

 

[TicklyT]

I've done a bit more poking around....... There's no banjo inside the case either, and it is fitted with a 60 Amp fuse.
I suppose a 45 Amp fuse would be more appropriate.
Something like http://www.tlc-direct.co.uk/Products/WYL45.html

Plus a banjo and a bit of 10mm earth wire.
........ and a pair of 25mm tails for completeness

 

[Spark123]

The 45A fuse would safeguard against future additions overloading the armoured cable. I take it you have a Periodic Inspection Report? What are the Ze and the PSCC values at the supply origin? What size/type of fuse have you got for the origin. No notes on main equipotential bonding if applicable or are they connected into the consumer unit?

 

[TicklyT]

The supply fuse is BS 1361, 100A.
Ze = 0.10 Ohm.
PSCC? Prospective Fault Current = 2.3kA.
10mm bonding runs from the CU through the ceiling to water and gas. (plus another 10mm to the suplimentary bonding in the bathroom that's clamped to the rising main 2 Metres directly above the main bond at the stopcock on the same piece of 15mm pipe )

 

[securespark]

Supplementary doesn't go back to the board/MET.....

 

[Lectrician]

What fuse rating does the PIR say you have protecting that SWA? Did you look at the actual fuse or just the carrier?

Using a 60amp Sw Fuse like that is OTT for a 6mm submain!

16mm Tails are absolutely fine. It is often a trick question raised by NIC area engineers, so not sure what the guy is on about above - maybe an inconsistant engineer.......not sure why an earth fault would make any odds to tail size.

The tails from meter to henleys have a little insulation showing - the tails should be ID'ed with tape or sleeve, not by removing the sheath.

 

[TicklyT]

I have checked, and it is a 60 Amp fuse cartridge in the carrier, but that didn't seem to concern the spark who did the PIR.

I think he may have slipped up here, as he has listed the conductor size of the SWA as 10mm on the PIR report.

The outside diameter of the SWA cable is consistent with both 10mm 2 core (17.4mm Dia according to the TLC website) and 6mm 3 core (17.9mm).
I have measured it as 55.5mm circumference ( / Pi = 17.66 Dia) and the cable is a 3 core.
The markings on the cable sheath are not easily visible anywhere, but it
isn't as large as the 10mm 3-core (18.9mm). It may turn out to be XLPE instead of PVC, but that still won't be rated at 60Amps.

I will have another poke around to see if I can find any legible markings on the cable sheath.
I certainly don't fancy poking about inside the switch fuse or second CU to accurately check the conductor size until I can safely isolate them, which won't be today for domestic reasons.

Securespark:- I didn't think the supplimentary bonding should have been run back to the MET, hence the
I am aware there have been discussions about whether doing so renders the installation more or less safe.
In this case both bonds terminate on the same length of 15mm pipe, without any joints in the pipework in between.

 

[TicklyT]

Panic over!

Finally found the cable is a BS 5467 10mm 3 core. From it's overall size I had assumed it was a 6mm 6943.

To assume = a way to make an ass of u and me

To recap:-
The service head has a 100 Amp BS1361 fuse.
There are 25mm tails from meter via Henleys to the first CU.
There are 16mm tails from the Henleys to a 60 Amp switch fuse.
There is approx 4 Metres of 10mm 3 core SWA to an RCD protected second CU with one 6A and one 32A MCB.
There is no connection to the cable armour other than the gland mounted in the steel switch fuse and CU enclosures.

How does that sound now?