OK the modern volt meter is likely a comparator same as the valve volt meter. But I am sure someone wrote a law about the relationship between volts, current, and resistance, OK I know it is AC so not a resistance but an impedance, but that is just splitting hairs, the ELCB-v used voltage to cause it to trip, and it seems they are returning in some form, to detect loss of PEN.
What I am aware of is
@CountryFan may be completely confused about this discussion.
There are many things I learnt in collage and university which I never needed or only referred to once in my working life. In fact some which I never did work out what was allowed, like the let through current of a fuse.
We get our meter and measure the loop impedance which tells use what the short circuit current would be, in the main we want it low enough to trip a MCB/RCBO within a set time, but most MCB/RCBO etc, will have another marking, energy let through peak 4.7 kA for example, so 230/4700 = 0.049Ω, and it is rare to find it that low, only once did it happen to me, and it was clear I needed to do something, which I thought a fuse before the RCBO with a let through valve lower than 4.7 kA was the way to go, but I was by no means sure. However, a fuse has some resistance so after fitting the loop impedance was over 0.049Ω, so the RCBO was now within limits. So never really looked into the let through value any more.
It was a tower crane used at Heath Row terminal 5, so it is unlikely
@CountryFan we ever need to work it out either. In the same way, unlikely he will even need to use imagery numbers. Maybe needs to know they exist, but not to use them.
But things do move on, go back 20 years, and with domestic we never saw a three phase motor, but today in my house I have at least 3, a fridge/freezer, a freezer, and a washing machine, possible the drier also has one? Last century we did have inverters, I remember getting one out of an old Pye Cambridge radio, a rotatory type, and that was how the old Lincoln welding bullet was basicly a rotatory inverter, but today we have static inverters everywhere, we don't need to repair them, but we do need to understand what they are.
I would attempt to repair power supplies, until the switched mode power supply came out, and one has to understand ones limitations, and I know switched mode power supplies and inverters are beond my ken to repair. Us them OK, but repair them no. Well OK I have dabbled, but the point is, we need to know when to say no.
The Part P regulations cause a problem with that. Go to repair something which one thought one could do without the need to register the work, then find it would need registering, so one needed to know someone who could take over those jobs, as the cost of using the LABC at £100 plus vat could really bump up the price. This is why I don't do domestic.
