Voltmeter on 3P CU - connecting very small wires

naylorpd · Nov 17, 2018

I'm in Azerbaijan so UK regs don't apply but safety and common sense do.

My house has a 3 phase supply to the main CU, which then feeds 6 other CUs around the house - one on each floor, one for aircon and one for garden. The incomer is 3P+N, no earth. I have two earth rods in the garden.

We sometimes suffer voltage drops, so I decided to put three panel voltage meters in the CU front panel so I could see what was going on. I intended to just use a spare 3P breaker on the CU to connect them. Trouble is, the breaker takes cables up to 10mm (at least) and the wires for the volt meters are a few strands thinner than a human hair. How to make the connection?

Thanks

Pete999 · Nov 17, 2018

What are these thin wires rated at, voltage wise, and have you connected them phase to phase, or phase to N?

naylorpd · Nov 17, 2018

The meters are rated to 500V and were connected Ph-N. Right now they aren't connected. The wires don't have anything printed on them but the insulation feels quite thick compared to the wires. I'm not worried about the rating of the meter.

Silly Sausage · Nov 17, 2018

I'd give them a good tinning and solder them onto thicker wire, and cover in heat shrink.

Pete999 · Nov 17, 2018

naylorpd said:
The meters are rated to 500V and were connected Ph-N. Right now they aren't connected. The wires don't have anything printed on them but the insulation feels quite thick compared to the wires. I'm not worried about the rating of the meter.
View attachment 45808

You should be able to connect these meters with a larger CSA cable and connect to the small meter wires by utilizing WAGO connectors with a suitably rate OCPD

Pete999 · Nov 17, 2018

Archy Styrigg said:
I'd give them a good tinning and solder them onto thicker wire, and cover in heat shrink.

Yes another very good way of doing it!

freddo · Nov 18, 2018

That looks like rubbish copper clad aluminium wire. Not much you can do with that other than remove the existing wires from the PCB and fit proper wires. Though I would not like to fit any product to a mains supply that the manufacturer thought it was OK to fit that type of wire.

Lucien Nunes · Nov 18, 2018

The meters may be rated to measure up to 500V, but are they rated for the overvoltage category applicable to a main DB? If so, I would expect to see better cables. If not, in the event of an incoming transient there could be some loud bangs.

I would get three suitably-rated inline 5x20mm fuseholders , connect the meters to one side and some 1.0mm² installation cable or tri-rated switchboard cable to the other side to extend to the MCB. In the fuseholders I would put low-rated (e.g.0.25A) ceramic bodied, sand filled HRC fuses. Maybe even lower. Must be HRC if they are to give you useful protection against exploding meters!

naylorpd · Nov 18, 2018

Lucien - some useful tips here.

The meter wires actually plug into the PCB so I may have the opportunity to upgrade them
Using inline fuse holders avoids joining two sizes of cable directly, so that's a plus. I like that idea.
Fuses seem OK, I can find HRC ceramic fast-blow 0.25A, but no idea if they are sand-filled.
"Suitably rated" fuse holders are a bit of a challenge. Bulgin FX0380 may be OK, rated at 250V (if inaccessible) but high voltage proof test is 2kV for 1 minute. And the IR is >100,000 meg at 500V DC. My mains voltage issues are more like spikes and brownouts than sustained over-voltage so this should do. Actually the mains problems are a lot less than a few years ago.

Thanks

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Voltmeter on 3P CU - connecting very small wires

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