"Rob Horton" wrote in message
...
Andy Pandy wrote:
"Rob Horton" wrote in message
...
Andy Pandy wrote:
How accurate are these? Just bought one - it's one with a magnetic
clip
which goes around the live load cable coming out of the meter. It
seems
to
read about 20-30% under at low usage (500W) compared to the meter, but
more
accurate at higher usage (2-9kW). I've tried another one which gives
the
same results. Could I have a faulty meter? Or do these things usually
read
under at low usage?

TIA


The problem is that these devices all use split core CT's (Current
Transformers). The splitting of the core will produce inaccuracies. No
matter how good the joining of the core, there will always be small
gaps
between the two surfaces which will affect the flow of magnetic flux
within the core. The reason for using them is that they are very easy
to
fit around existing installations.

The best solution is to have a non split CT. You would have to isolate
your mains supply, disconnect the live tail, poke it through the CT and
then reconnect the live tail.

It could also be a phase difference issue. Are you using the same type
of load to produce the readings at 500W and 2KW-9KW?

No - 500W was mainly lights, telly, fridge, freezer etc.

2-9kW was kettle, shower.

Sooo, making some assumptions here, lights could be CFL?

Mostly.

TV - maybe has
a switch mode power supply?

Not sure. How do I tell? It's a fairly new flat screen (probably about 2
years old). It takes a few seconds to respond when switching on.

Were the fridge & freezer actually running
at the time?

I think so but not sure.

Kettle - simple heating element & shower - electrical heating with no
motor?

Yup - no motor.

I'm not saying that this is definately the cause just pointing out the
differences.

OK thanks.

I would like to get a remote monitor and have wondered how accurate they
are.

How do you know it's the monitor and not the supply meter that is wrong?

I don't - that's my main concern! I may have been overcharged for 10
years...but before taking it up with the electric company and having them
laugh at me I thought I'd check whether it's usual for these monitors to be
so far out.

As it happens they've just written to me saying the meter needs changing...

--
Andy