Acceptable maximum supply impedance?

A friend asked me why the lights in his house always seemed so dim. A quick investigation found that below spec voltage was the primary cause since although getting something in the region of 230V mid morning, it fell to about 205V at 7pm. This is obviously below the minimum threshold and probably worthy of complaint to someone.

However, some more testing also revealed that the supply impedance was also pretty poor at the house - getting on for 0.5 ohms in fact. Hence at maximum load he could be looking at a supply voltage of just over

100V! (not that you would eve be able to reach 100A loading given the available voltage).

I also noticed that the supplier had fitted a meter rated at 40A after the 100A main fuse. How common is this?

The supply looks like it starts life at a power pole with its own transformer supplying about four houses. The last few meters of cable running underground before it reaches the house. House earthing is PME, and this is a reasonably recent install - less than 15 years old.

Reply to
John Rumm
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On Mon, 02 Jan 2006 17:07:33 +0000,it is alleged that John Rumm spake thusly in uk.d-i-y:

Agreed, at 230v +10%/-6% the minimum should be 216 or so.

Very. Our house is on a 100A main with a 20~60A meter.

I am suspecting a bad joint somewhere on the cable, the danger could be that it may be on the neutral, and with PME this is _not_ a good thing.

Being that it's outside statutory limits, and there's also the potential for a serious hazard, I would think the supply authority should get someone there ASAP.

Reply to
Chip

PME (TNC-S) Ze 0.35 ohms

TNS Ze 0.80 ohms [max values quoted by LEB usually] but that is only on paper!

Ring them up and complain, probably won't do any good, but...

40A meter!?, they just seem to fit whatever they have on the back of the horse and cart! [up here they use carrier bikes and handcarts]

HTH

Reply to
Grumpy owd man

I was talking about the supply impedance (i.e. phase to neutral) - not earth impedance.

(I can't remember exactly what I measured that at (might have been 0.17 ohms) at a cooker point 5m from the CU on a 4mm^2 T&E - but it was well under nominal 0.35)

Reply to
John Rumm

Would not Ph - E and Ph - N be the same on TNC-S? after all they are derived from the same place in the service head, are'nt they? Or am I still recovering from a rather nice Merlot on New Years Eve.

Reply to
Grumpy owd man

Yebbut for PME, Ze is the same as the supply impedance.

I think 0.35 ohm is the "highest normally expected value" rather than a guaranteed limit.

I'd suggest putting in a low voltage complaint to the supplier - see previous posts from Colin Wilson and 'Wanderer' for guidance.

Reply to
Andy Wade

If the supply is PME then a call to relevant REC (DNO) emergency number is appropriate as there is a risk of the neutral failing and causing all sorts of damage.

Steve Dawson

Reply to
Stephen Dawson

Mmm, yup good point... I did not think that one through to its logical conclusion as I typed it!

My assessment of the supply impedance was calculated with a number of readings of voltage Vs load measured on a ring final circuit close to the CU. The earth impedance I quoted was what I measured with my megger LT5 (although at a different time of day it has to be said).

Something does not quite add up! (if nothing else it sounds like I need to go check the resistance of the ring circuit in isolation!)

Yup, me thinks that is what I will suggest he does.

Reply to
John Rumm

The LT5 is a reliable beast and usually give the right answer (well mine seems to ...), so I suspect that either (a) it's all down to the resistance in the final circuit, or (b) there's an intermittent bad connection somewhere.

A tip for supply impedance measurement by the voltmeter-and-known-heavy-load method, is to connect the voltmeter to a different final circuit to the load, thus eliminating the effect of any impedance after the CU busbars.

Reply to
Andy Wade

(a) I can test and eliminate... (b) is an interesting one - if it is not all down to (a) then it is almost as if there is a non linerarity in the supply and its impedance rises noticeably under load (shared between properties rather than just in one).

Yes good point - did not do that last time - I had a clamp meter round ten turns of my "designed to make accurate clamp metering easy" extension lead, but I was reading the volts as the same socket it was plugged into.

I shall repeat the load test again I think... for my own satisfaction if nothing else (not that it makes that much difference - the main issue is lack of volts to start with before you go adding load!)

Reply to
John Rumm

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