Strange lighting wiring

Probably. But it's certainly not 12 volts :-)

Induced voltage from parallel conductors.

With cables having coupling in the order of about 100pf/metre (minimum), it only takes 10m to get a whole 1nF (i.e. 73uA leakage). Digital multimeters (good ones, that is) are about 20Mohm input. If you measure with a good ol' analogue meter you'll probably find a lot lower than [76V].

Looks right. Cable capacitance plus a modern high impedance meter - use an old lower R one and those stray Vs will fall away. No earth conductor though?

NT

The meter is an old (circa 30 years) fluke meter. Good quality but old. There is earth at the junction box - should I run some more reading with that?

A 30-year old digital meter will probably be 20M or more, while an older, analogue meter might be 10K.

SteveW

Some Fluke's like a few of the 100 series have a special low impedance range designed to combat this kind of phantom reading. When selected the input impedance drops to around 3K.

Decent moving pointers were 50k/volt, so 15M on a 300v dc scale, half that on ac. Rock bottom ones were 1k/v so 150k for 300v ac scale.

NT

Well I never knew before that the Avo 8 (with 1000 ohms-per-volt on AC ranges of 100V and upwards) was at least close to "rock bottom" in the moving coil meter market.

If your meter is one of these cheap"mickey mouse" ones. they often give spurious readings. It's on account of the tiny current they draw.

Generally was lower on AC ranges. For DC, was 20,000 ohms-per-volt.

I have one of the last ones manufactured.

Had tis on a complex circuit: seemed to be from an octal relay, but only about 230V on a DMM. AVO 8 showed about 200-odd, dropped the range and the figure went down, got to lowest range and bit above FA there. Slightly bent fixed contact, little tweak and it was OK.

Indeed - which is still far short of "decent" (as defined above).

Mine was made in the 60s - and the leads are rather inflexible as a result of the copious tape where the insulation has perished. (Me too mean to buy new leads for what is very, very occasional use.)

Yes I'm assuming each light has its own transformer and the wiring is thus at mains volts. Not sure about the odd colours though. What one needs is a tungsten lamp on a bit of wire. Beats a meter for this stuff any day! grin. Brian

grey, black and brown is the new blue, yellow and red for three core cable.

I used to have an old TMK analogue meter that had a button that actually put a fairly large wattage resistor across the probes so you could see what actually happened under a minimal load, and it used to be very surprising on even fairly short runs of wire at 240v ac.

Brian

The Avo 8 is something from the 1930s. A lot changed since then thankfully. 1k/volt is precisely what rock bottom analog meters from the 60s on were. Rock bottom in the 1930s was the watch type moving iron meters, much less than 1k/volt.

NT

That's not even wrong...

With analogue meters, the better the quality, typically the higher the input impedance. This is in the majority of measurement circumstances a desirable characteristic since it reduces the parasitic effect of the meter on the circuit under test.

With digital meters the input impedances are typically significantly higher. Again in many cases this is a "good thing", although it is a trap for the unwary with "floating" circuit cables in close proximity to mains voltages.

An ideal meter would draw no current at all when measuring voltage.

The AVO 8 came out in 1951 and stayed in production until 2008

When I started work in a TV service department in 1960, all the meters were AVO 8 models. All service data quoted 'all readings taken with 20k/volt meter', which is what the AVO 8 was.

However, looking at info for sets we were still repairing, the voltages had obviously been taklen using its predecessor, the AVO 7.

This points to the AVO 8 being introduced around the mid 50s

The above relates only to the DC ranges, of course.

The AVO 8 was regarded as the top of the range and had a price tag to match - certainly well out of my league.

My first meter had a similar specification but not the mechanical robustness, nor the 'bombproof' cut-out protected movement of the AVO but it served its purpose well. I may still have it around somewhere.