[chbot] Electric power meter safety concerns...

[Helmut Walle]

Yes, I have received the same from Meridian a while ago. It's not 
necessarily about the equipment, but it may simply be a combination of 
the seasonally colder weather, plain physics, and house wiring architecture.

As the weather can be cold at this time of the year people will use 
heating appliances that can significantly increase power use. Let's take 
an old poorly insulated drafty 3-bedroom house and put one 1.5 kW oil 
radiator in each bedroom, plus a nice big 2.5 kW in the lounge. On a 
cold winter night they are all blazing away at full power. When the 
night rate kicks in at 9 pm the hot water cylinder adds another 3 kW of 
load. At the same time someone is cooking dinner - 1-2 kW for the range. 
Slice of toast? 1 kW. Cup of tea. 1 kW. After showering, someone uses a 
hair drier - another kW. Where are we now? At about 15 kW, which 
requires 62.5 A of current at 240 V.

Now all of that current obviously needs to go through the wires from the 
distribution network to the meter, and through the meter into the 
switchboard, from where it splits up into whatever circuits the house 
has, and each circuit will only carry some fraction of the total current 
going into the house. The wire from the power line easily handles the 
total current and more. The circuits inside the house have fuses or 
circuit breakers to protect them. And between the distribution network 
and the meter you have a pole fuse or circuit breaker.

Most single-phase residential connections around here have a 63 A fuse 
or circuit breaker. Fuses and circuit breakers have slightly different 
tripping characteristics, but they all respond to rather large 
over-currents very quickly, while at the same time showing a fairly 
relaxed attitude to moderate over-currents. You obviously want to be 
protected against major "explosive" damage, but you don't want the fuse 
to blow each time you go over the rated current for a few seconds.

The vast majority of single-phase electricity meters in NZ can handle 
100 A indefinitely, and they can usually withstand even higher currents 
for a little while without suffering damage. So the meter is not going 
to suffer any damage at all if you are running 100 A for ten minutes. 
(Typical 63 A fuses and circuit breakers will not trip that quickly 
either.)

So far, so good. However, there are wires going into the meter, and out 
of the meter, and these wires obviously also need to carry the whole 
current. The wires themselves are not usually causing any trouble unless 
they are damaged. If there is going to be an issue regarding excessive 
heat then it will usually be at the points in the current path that have 
the greatest resistance per unit length. These points in any electrical 
wiring system are often the connections between the various parts of the 
system. The wiring terminations into, and out of, the meter are 
therefore of critical importance. If the terminations have been 
completed according to best practice, and if they have not degraded over 
time, there will be no problem here. However, if the wiring has been 
completed in a sub-standard way, or if the terminations have degraded 
over time, for example due to oxidisation, then the greater electrical 
resistance at these points can cause excessive heating that may lead to 
damage, or that in the worst case could start a fire.

The problem will obviously only be an issue in practice if, and when, a 
sufficiently high current is passing through these points of excessive 
resistance. And the most likely time for this to happen is in winter, 
when loads of heating appliances are causing much higher currents to 
flow than during the warmer months of the year.

Hope this helps to explain the concern. If you follow the "Find out 
more" link from the Meridian message you will see that Meridian are 
covering a fair range of safety questions on the respective page:

https://www.meridianenergy.co.nz/power-outage/staying-safe

Most electricity users are probably not aware of the physical and 
technical details of electricity in detail, and being aware of hazards 
is a big part of keeping everyone safe. Good to see that Meridian are 
doing something to make their customers aware of the risks, and how to 
respond if something happens.

Have a great afternoon!

Helmut.

On 09/08/2024 11:17, Mark Atherton wrote:
> Just received a most interesting note from Meridian:
>
> "Electrical meter safety warning: We want to keep you and your family 
> safe around electricity. If your meter is smoking, has a burning smell 
> or catches fire, call 111 and ask for the Fire Service. Do not attempt 
> to put an electrical fire out"
>
> Don't think I have ever received a notification from any utility ever 
> with safety concerns about their own equipment.
>
> Interested to know any associated back-story. It could of course be 
> their legal department being cautions ??
>
> -Mark
>
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