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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.
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.
The voltage might be just within tolerance at 40 amps.
Nice observvation on your part
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.
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?
I am not sure that it is fair to blame the DNO when large (very large) loads are subsequently installed.
Before complaining though, do try and ascertain what the agreed supply capacity is.
So the DNO have confirmed a 100-amp supply.
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.
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.
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.
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.
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!)
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?
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.
. . . 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. . .
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
... but even with binoculars unable to determine its rating. ...