Old EON clamp meter - any use?

It would be trivial to assume a sinusoidal waveform and do the calculations.

The source of mains is very low impedance, therefore there is no need to sample Voltage as accurately, or indeed at all if you're willing to make some rather obvious assumptions.

Nonsense.

Unfortunately he is right, if not entirely for the right reasons. The issue is not with the voltage *waveform*, which may well be a fairly pure sine wave despite the household's non-unity power factor. In order to distinguish reactive from resistive current you need to know the phase relation between the voltage and current. This can only be done by having a voltage signal as well as a current one.

Yes, I can see I said nonsense to his post in general.

TNP himself agreed that "Very little household load is inductive". Typically current is nominally in phase with voltage, the exception would be crude dimmers but even they have established crossing points. Many devices conform to legislation to ensure current is in phase to reduce the effect of harmonics fed back into the supply.

In short, most of the time current vs voltage is usually in phase. Most crude energy measuring devices agree fairly well with actual consumption.

I fully accept that for accurate measurements then an voltage waveform needs to be known, but much can be inferred from the current waveform.

For a household load, yes, but the suggestion was that you use this on individual circuits to get more granular readings than your smart meter produces. I don't believe that legislation covers devices at low power, so in standby no PF correction is required. This is when we see results where the above isn't true. So look at this thread:-

This results in suggestions that you can save money by switching things off over night when in fact you won't.

So at 1.4 watt I think that is 1kWh per 30 day month, so on the price cap 24.67p per month, or £2.96 per year.

Given that for any device produced post 2013 the limit is 0.5watt if no display, 1 watt for devices with a display, you need to switch a lot of devices off to make a significant saving.

As I said, the problems occur on edge cases where these devices are frankly crap...

Dave

It might be -but it is also not uncommon to see some distortion in the voltage waveform - typically the peaks getting flattened somewhat by the abundance of switched mode loads.

With many modern small loads the current load is very spiky and not at all sinusoidal - so it is difficult to establish a traditional phase shift as such.

That only applies to SMPSUs over 25W IIRC. So you computer PSU will probably have decent PFC, but all those other small loads, electronic gadgets, and LED lamps will often have a very poor PF.

I find with my "quick hack" power monitor[1] (which computes the instantaneous product of the current and voltage waveforms) it sits at

The problem is that with so many SMPSUs around it is nothing like a sine wave any more.

And not just some assumption of sinusoidal waveform,. Hence why you need to know the instantaneous value of voltage AND current and make no

Which is what I said

Ultimately you don't need to know the *phase*., Phase only applies to

And we have neither, in current nor voltage

It's the same as the old stupidity about 'rolling radius' in a tyre under load. It isn't round, It doesn't have a radius

It has a circumference,

Non sinusoidal waveforms can be considered to be, by use of a Fourier transform a harmonic series of sinusoids all at different frequencies and in different phase relationships, That's way too complicated to calculate when you have the simple method of sampling both waveforms, multiplying the instantaneous sample values together and averaging the results over a cycle

In the same way that slinging a tape measure round the rolling tyre is simpler than trying to integrate for a slew of different virtual radii, to get the circumference which in the end is what you need to know.

Electrical theory as taught to O level is very simplified. The real world is different

Yer-but. While it will have a poor PF that won't be from a reactive load, but one that only conducts at the main's peaks. From that it wouldn't be difficult to determine the crossing points.

The article doesn't make it clear precisely what was being measured at

I doubt his meter measures true rms.

I found that with my (inverter) microwave. The large filter capacitors in the mains filters take a purely reactive current.

I agree that's the right way to do it, and not too expensive these days. The main cost is probably making hardware to suit safety standards while measuring

But the original question was whether a current transformer on its own would be useful, and we seem to agree not very useful.

Going into the recycling.

Cheers

Dave R