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Old 17th Aug 2021, 8:57 am   #1
colourking
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Default 1970s Wylex double-pole isolator switch M131/2

Pictures attached of a Wylex double pole isolator switch which was between electricity meter and domestic fuseboard, installed in the earlier 1970s

Under the cover the switch has model: M131/2 and M131.
left side of switch is indicated N, right side is L
Appears installed with 6mm wires.

Anyone know the ratings these switches had?

Obviously it was crazy for the installer to use red wire for both Live and Neutral. Maybe they had ran out of black.
Invited the likelihood of mix-ups !
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Old 17th Aug 2021, 10:50 am   #2
duncanlowe
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Default Re: 1970s Wylex double-pole isolator switch M131/2

It's also possible, that they were switching only L, but used two wires and both poles, to increase capacity.
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Old 17th Aug 2021, 11:15 am   #3
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Default Re: 1970s Wylex double-pole isolator switch M131/2

One, (or maybe both) of the short tails between the cutout and the meter on my original electrical installation, were two wires in parallel.
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Old 18th Aug 2021, 9:00 pm   #4
colourking
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Default Re: 1970s Wylex double-pole isolator switch M131/2

Quote:
Originally Posted by duncanlowe View Post
It's also possible, that they were switching only L, but used two wires and both poles, to increase capacity.
No, it was switching both L and N (confirmed), but using red wires for both!
The wires went into holes to behind the board re-emerging in the fuse board, and at the meter at the supply end.
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Old 18th Aug 2021, 9:50 pm   #5
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Default Re: 1970s Wylex double-pole isolator switch M131/2

I hope your mains supply has something a bit beefier than 6mm2for the feed from the meter I suspect you mean 16mm2 which is more like it
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Old 19th Aug 2021, 8:37 am   #6
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Default Re: 1970s Wylex double-pole isolator switch M131/2

16mm2 is bigger than welding cable, not ideal for any house stuff. The OP is likely to have used a vernier to measure it, it will show In the region of 6mm measured across.
Shower cable is supposed to be 10mm but that is big enough to start a lorry engine!
It would be advantageous to know exactly how it is being measured.
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Old 19th Aug 2021, 8:39 am   #7
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Smile Re: 1970s Wylex double-pole isolator switch M131/2

Hi,
I have a couple of those in my junk box, and the rating is stated on the back. I think that the one I have is 45 amps, but I'll have a look later.
Cheers, Pete.
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Old 19th Aug 2021, 10:20 am   #8
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Smile Re: 1970s Wylex double-pole isolator switch M131/2

Hi,
Just had a look at my Wylex switch, and it's rated at 60 amps.
The no. inside the cover is just the mould no.
Here's a view of the reverse.
Hope this is helpful.
Cheers, Pete.
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Old 19th Aug 2021, 11:14 am   #9
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Default Re: 1970s Wylex double-pole isolator switch M131/2

I remember we had something very similar to that - but also fitted with a neon - for our immersion-heater control.

[It was a rather powerful immersion - 6Kw if my memory hasn't faded]
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Old 19th Aug 2021, 11:25 am   #10
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Default Re: 1970s Wylex double-pole isolator switch M131/2

We had one without the neon for the immersion located next to the fuse box.
Indication that it was ON was by a red 15W pigmy bulb in a batten holder. It would have been that way from when the house was built in 1953
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Old 19th Aug 2021, 5:24 pm   #11
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Default Re: 1970s Wylex double-pole isolator switch M131/2

6mm diameter (of the copper) is close on 19mm sq. 6mm diameter including insulation is probably more like 4mm copper (not sure how thick the insulation is) which is just over 12mm sq.

Meter tails seem to be 25mm sq these days. As this seems to have been used before the CU, it would need to be substantial to ensure it would open the 'company fuse'. So 6mm copper diameter seems reasonable. 6mm sq would worry me if there was no fuse between this and the meter.
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Old 20th Aug 2021, 10:31 am   #12
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Default Re: 1970s Wylex double-pole isolator switch M131/2

AFAIK these switches used the same components as the switches in their consumer units, of which the common types were rated at 60A but there might have been a 45A version as per @Tractorfan (or was it 40A?) There were other, similar-looking versions such as the immersion one and 3-pole metalclad 20A type with a wider toggle, some of which we still have in use here in the workshop

6mm or near imperial sizes were not unusual for meter tails especially in the days when 40A cutout fuses were common. In fact two single-core 6mm are more or less rated for 60A and many cutouts are still 60A today. However the convention is now to install 25mm allowing for future 100A service upgrades without having to change the tails.

Depending on whether the total load served by the tails exceeds their contiuous CCC, the service cutout fuse upstream may only ever need to provide fault protection, not overload protection. For example, an additional CU with total outgoing In (OCPD nominal rating) of 50A in theory only needs a supply cable rated for 50A, provided it meets the adiabatic requirement to avoid thermal damage at the prevailing fault current. This can often be achieved with what appears to be a remarkably small cable, compared to one rated for the full upstream OCPD In.

Taking a modest PSCC of 1kA and 70 PVC insulation with a k constant of 115, and a 60A BS 1361 fuse that achieves <0.1s disconnection at 1kA, the minimum meter tail CSA that would be required in order to be protected against short-circuit by the cutout fuse would be:

CSA = sqrt(It)/k
= sqrt(1E+6 * 0.1)/115
= 2.8mm minimum.

For that reason, old small tails feeding individual switchfuses, as were commonly installed to serve a 30A ring final circuit when socket-outlets were modernised to 13A, have often remained safely in service. 7/.044 was common, or 7/.064 for a larger consumer unit.
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Last edited by Lucien Nunes; 20th Aug 2021 at 10:42 am.
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