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Minimum Voltage at incoming supply point
FlyWheel
40 Posts
Question
Retired now and not up to date with reg’s since 16th. Helping advise a friend. Currently they have 100amp single phase supply and whilst the no load voltage is up at around 238 volts, as the circuit is loaded, voltage drops off substantially. Even with a load of 50 amps, ie, half the supply fuse rating, voltage is slightly below the -6% limit, at around 215 volts, further load simply pushes voltage significantly lower, and by extrapolating measured data, it would be nearer 170 volts with a load of 100 amps, is this acceptable? I plan measuring Ze and pfc this week as a guide perhaps to supply loop impedance and, which I suspect must be causing this excessive drop. I’m aware of the ESQCR reg’s although need to update myself, but are there any other regulations that apply?
 
111 Replies
broadgage
367 Posts
It sounds unacceptably low.
Before complaining though, do try and ascertain what the agreed supply capacity is. It might be a lot less than is implied by the 100 amp cut out fuse holder.
Have you seen the fuse rating ? It might be a lot less than 100 amps. Or the fuse fitted might be larger then it should be.

If the supply voltage is persistently below the minimum with the load not exceeding that agreed, then there would seem to be cause for complaint to the DNO.
I presume that the voltage was measured at the point of supply, or reasonably close thereto and not at the far end of a long or marginally sized customer owned sub main. And that the instrument used is preferably calibrated recently.
FlyWheel
40 Posts
Thanks Broadgage. And yes, I had been wondering about the supply being limited to below the 100 amps  (but even if only 60 amps, voltage is still out of spec) of the intake fuse currently fitted, and I will be seeking to clarify this.  (it’s a very long run to the supply transformer, with old lead sheathed cables too, so likely to be TN-S). Yes, voltage was measured at supply input. DNO engineer has very recently collected a weeks worth of site voltage and current data, and in my opinion, has misinterpreted the data , and I will be strongly questioning this. First though, I am meeting a particularly reliable electrician on site today (one of few that I could rely on during my time as a Council Electrical Contracts engineer/manager, one of my "hobbies" after retiring early from main engineering career) to use his recently calibrated tester, to measure Ze and the pfc, and I’ll get him to also compare measured voltage with that recorded by my multimeter used (although not recently calibrated, I do regularly compare it with some of my other multimeters, to get confidence in the indications). 
 
mapj1
2348 Posts
Welcome.

That is very droopy!  20V -25V drop for 50A load is  about 0,4- 0.5 of an ohm.  You would have trouble convincing me that any large trips (cooker circuit, shower)  will  be sure to meet the prompt disconnection requirement if there is a fault,  without help from an RCD unless the indoor wiring is very short. (we like to see Zs low enough to be sure of a fault current of rather more than 5 times the breaker rating to  ensure a rapid break, in this case most of the Zs allowance seems to be used up outside.)
Still it means you will be able to easily see the kettle being plugged in, as the light will dim visibly. 😀
An additional test, if you have the patience, would be to look at the L-E drop and then the N-E rise when a known load is added and removed (that kettle would do).  That will tell you if N and L are both undersized but the same resistance, suggesting a thin cable that has always been wrong, or if there is a higher resistance in only one or the other - more likely to be a fault. (And if you are not sure if it is really TNS or PME in the street presenting as TNS, which is quite common after repairs or additions to the mains, then the CPC earth to true terra-firma earth voltge will bounce with load if the NE link is on your side of the neutral voltage drop.
Are other buildings on the same substation affected or is it just this one ?

Older buildings may have originally  had a 'lighting only' supply that has been modernised with only a new cut out fuse, rather than a new cable.
broadgage
367 Posts
It is possible that the supply is restricted to only 40 amps. Or is MEANT to be 40 amps and that someone has improperly fitted a larger fuse. Not very likely these days, but 40 amp supplies do still exist. I found a 25 amp service recently.
The voltage might be just within tolerance at 40 amps.
AJJewsbury
1764 Posts
Or could just be a loose connection somewhere. I was asked to look at one house where 'the lights dimmed when the kettle was switched on' - loop readings were all over the shop but the give away was the audible 'crackling' sound coming from the DNO's cutout when the kettle was on. A call to the DNO with "my electrician says..." apparently brought a swift resolution.
  - Andy.
ebee
776 Posts
Suresly AJJ when doing a periodic/EICR you use your six senses? I thought everyone put the kettle on and listened to the cutout! If not they will now.
Nice observvation on your part
Kelly Marie
293 Posts
Ebee and AJJ this reminds me that we had a loose cut out fuse here at Kelly towers it ran a bit warm and crackled with the kettle or immersion heater on I foned  the DNO who didn't really want to attend so I said can I cut your seals and do it myself oh no don't do that they said  one of there blokes was at the door in 15 minutes amazing isn't it? If you concentrate there minds service is quite good.
FlyWheel
40 Posts
Thank you all for you input, it’s helped a lot. I’ve also had the DNO confirm 100 amp’s approved, although how they can expect voltage to stay in limits with a supply cable impedance near the TNS 0.8 ohm limit, I don’t know. For the installation I’m looking at (for a friend, so just out of interest in my retirement), the current demand, by domestic standards is large, and includes heat-pumps for both house and pool, and these in particular are being adversely affected by the low voltage. There are implications too re the COP of heat pumps and lots more with at these low voltages, so I have just submitted a comprehensive technical report to the DNO and asked for an urgent response. I also have some questions about the accuracy of Ze and pfc measurements on a loaded network, using standard loop testers, but I’m currently considering that separately.
 
broadgage
367 Posts
With a confirmed supply capacity of 100 amps, it now seems clear that the DNO are failing to supply the required voltage and must correct this.
Heat pumps, being a type of refrigeration compressor are particularly vulnerable to low supply voltage, as are fridges, freezers, air conditioners, and de-humidifiers and similar equipment.

Many other loads will be fine. Resistance heating will give a much reduced output, but not suffer any harm. Modern LED lamps should be fine down to under 200 volts, with some types operating down to about 100 volts.
perspicacious
364 Posts
the current demand, by domestic standards is large, and includes heat-pumps for both house and pool, and these in particular are being adversely affected by the low voltage.

Has to requirement to notify the DNO of the wish to connect a "disturbing load" over 2? kW and the consequences of "flicker" been rescinded?

Regards

BOD
Chris Pearson
1659 Posts
BOD is on the money yet again. When I got my 3-phase supply, the DNO wanted to know why. They were interested in the use of motors, but accepted that 3-phase ones were less likely to make the lights flicker on start-up.

I am not sure that it is fair to blame the DNO when large (very large) loads are subsequently installed.
UKPN
152 Posts
Its a non post, the poster just wants the DNO to do the calculations, we get it all the time. This is clearly an additional load, and we require a declaration. Perhaps he can come on here and tell us the calculations he has arrived at. I doubt we will hear of those though.

Regards, UKPN
FlyWheel
40 Posts
Believe what you want UKPN, the response I’ve seen from the DNO is pathetic (I’ve only just taken up the case), and I have done the measurements, calculations and submitted a detailed report, and await the outcome. Have worked for the CEGB -power stations- that was a while ago!), and most recently, as a Council Electrical Contracts Engineer (a hobby during retirement), and even surveyed total plant and services for major NHS hospitals (some of them are in a mess!), and much more. Including as a drives engineer and controls specialist over a long career in the industry. I am very use to having to do such investigations, but not up to date with latest Reg’s (several years into retirement), hence the question on here, and appreciated the answers. Now sitting on it awaiting DNO response. Back in 2006, after initial early retirement, I also completed the C&G2391, not because I needed it, but because it looked interesting, and found in practice that some of it could be nearly as difficult as earlier degree and HNC’s exams, and back then, most failed it, but I think they have simplified it since.
 
Chris Pearson
1659 Posts

broadgage:
Before complaining though, do try and ascertain what the agreed supply capacity is.

Have we had an answer to this important question please?
Sparkingchip
2928 Posts

PFlyWheel:
Thank you all for you input, it’s helped a lot. I’ve also had the DNO confirm 100 amp’s approved, although how they can expect voltage to stay in limits with a supply cable impedance near the TNS 0.8 ohm limit, I don’t know. 

So the DNO have confirmed a 100-amp supply.

If you attach the loop tester to line and neutral I would expect a lower reading than around 0.8 ohms, however I am now picturing a nice house in a rural or suburban location with a long run of DNO cable supplying it that was installed many years ago when the maximum demand of installations would have been expected to be a lot lower, indeed this is probably the first time the system has been stressed.

Andy Betteridge 
 
perspicacious
364 Posts
I’ve also had the DNO confirm 100 amp’s approved

Poetic interpretation I expect of the answer to "what size is my cut-out fuse" rather than what is my Authorised Supply Capacity (ASC) which is typically 12, 14 or 15 kVA.

Most consumers don't like the quote of £8k? to upgrade their ASC or to allow "disturbing loads" to be connected.

Regards

BOD
Sparkingchip
2928 Posts
As it is TNS I am assuming the DNO cable is at least fifty years old and the house has probably been extended considerable since it was installed.

Andy Betteridge.
FlyWheel
40 Posts
Yes, confirmed 100 amp in writing following their voltage technicians data logging exercise, and in some earlier correspondence too.. The situation, which started with a request for a 3-phase supply very much earlier this year, when pool heat pumps were being considered, has dragged on for a very long time and I'm now looking for a quick resolution. I only got involved, casually, a few weeks ago. My report has apparent been sent to the DNO today.
perspicacious
364 Posts
So we've gone from considering to actually having them installed without getting the supply upgraded then. As before, it appears the cost of getting the supply upgraded was deemed unnecessary so the install went ahead and now the reasons for a paid for increase in ASC have come home to roost.......

 The situation, which started with a request for a 3-phase supply very much earlier this year, when pool heat pumps were being considered,

For the installation I’m looking at (for a friend, so just out of interest in my retirement), the current demand, by domestic standards is large, and includes heat-pumps for both house and pool, and these in particular are being adversely affected by the low voltage. There are implications too re the COP of heat pumps and lots more with at these low voltages, so I have just submitted a comprehensive technical report to the DNO and asked for an urgent response.

Regards

BOD

 
Please keep us updated as and when.

I've posted here re a friends supply where the DNO wanted a percentage of a transformer upgrade and cable costs to supply 3-phase, yet the offered single phase supply capacity to the existing house was going to be downgraded. For those who remember the post, it is still ongoing. Strangely the transformer was replaced (at no cost to my friend) but even with binoculars unable to determine its rating. (It supplies 4 houses and an agricultural building - which is where my friend wanted the 3-phase supply. (I even went to where SP store their new transformers and took photos through the fence where I could see rating plates for comparison purposes!)

Clive
 
perspicacious
364 Posts
The situation, which started with a request for a 3-phase supply very much earlier this year, when pool heat pumps were being considered,

The DNO have to provide a quote within a fairly short time period so have you not had the quote yet? If so, it is an administrative complaint for delay in providing a quote, or was the quote considered too high and the friend was "advised" by the supplier of the heat-pumps that everything would be OK on single phase?

Regards

BOD
Sparkingchip
2928 Posts
As I remember it if they are running a hotel or some other business which would make it commercial premises the time the DNO has to respond is increased considerably.

From experience I’m not surprised if it takes a long time to sort out and they may be surprised how much the upgraded supply will cost. They may have jumped the gun getting the installation work done.

Andy Betteridge 
Alan Capon
382 Posts

FlyWheel:
. . . The situation, which started with a request for a 3-phase supply very much earlier this year, when pool heat pumps were being considered, has dragged on for a very long time and I'm now looking for a quick resolution. . . 

As others have suggested, did the original quote for a supply upgrade not appear, or was the figure in pound notes not liked? 

If a supply upgrade was considered necessary and a quote asked for, it looks like the additional loads have been connected to the public supply anyway. Have the DNO been made aware of this, as I am sure most supply contracts need additional load notified, especially where potentially large motors are installed. The DNO also have the right to limit the number of starts per hour. To be honest, the presence of the pumps and their starting regime is likely to be obvious from the DNO’s recorder anyway

Regards,

Alan. 
Chris Pearson
1659 Posts

AncientMariner:
... but even with binoculars unable to determine its rating. ...

I found that I needed a telescope and a good firm tripod.

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