Discuss Volt drop, does it matter? in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

EricMark

EF Member
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Mid Wales
There are two items where volt drop can have a large impact, magnetic ballast fluorescent lamps, and refrigeration units, there are some other odd items like shrink rap machines, but most items today use switch mode power supplies so volt drop would need to be rather large before it becomes a problem.

However micro generation has resulted in tapping being changed in supply transformers to houses where for years the supply was 245 volt suddenly end up with 230 volt, so a on premises volt drop which has been there for years, now becomes apparent.

It is easy enough to measure the loop impedance of an incomer and loop impedance on a socket, put the results into software and see the volt drop, however with a tolerance of 0.02 ohms on both readings then to claim the volt drop is exceeded presents a problem.

OK if the volt drop is 20 volt one can be reasonably sure it's not a reading error, but at what point if ever would you highlight a large volt drop? And if a contractor has made alteration which cause a volt drop and you don't highlight it, if it causes a problem latter could the owner claim from you?
 

davesparks

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How exactly are you using loop impedance measurements to establish the volt drop? How are you including the load current into your calculation?

Yes volt drop does matter even if the load can handle a lower input voltage, the volt drop also affects the energy lost through heating of the cable and the potential between N and CPC at the end of the circuit.
 

UNG

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However micro generation has resulted in tapping being changed in supply transformers to houses where for years the supply was 245 volt suddenly end up with 230 volt, so a on premises volt drop which has been there for years, now becomes apparent.
Not sure where you get your facts but the voltage reduction to 230v +10% / -6% was a result of a 1994 EU directive to harmonise voltages across the EU and not micro generation as you suggest. Due to network equipment costs the voltage reduction across the network is an ongoing project that will carry on for many years. Back in the 90's at a Manweb trade event, a presentation was made on the subject and I seem to recall they were looking at an engineering cycle of around 40 years to bring the voltage down to 230v with scheduled equipment updates and that was just the Manweb region
 

davesparks

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Due to network equipment costs the voltage reduction across the network is an ongoing project that will carry on for many years.
It's actually being done? I was under the impression that the change was only a paperwork exercise and never going to actually happen in the real world. I thought even the newest substations were still outputting 433/250?
 

UNG

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Nr Wigan, Lancs
It's actually being done? I was under the impression that the change was only a paperwork exercise and never going to actually happen in the real world. I thought even the newest substations were still outputting 433/250?
As I said Manweb's policy back in the mid 90's was to do nothing immediately across the network but as equipment was replaced or newly installed then they would reduce the voltage that might have changed when SP Energy Networks took over . Over the last few years I have come across a few places that have had noticeable drops in voltage close to where work has been carried out on the network with area where the voltages were as low as 216v

The other reason for the gradual changeover was that the voltage harmonisation was to be in 2 parts initially it would drop to 230v and if I remember correctly in the late 90's / early 00's is was supposed to drop to 220v, the last I heard of the drop to 220v from a DNO engineer was that it was still under consideration and due for discussion in the next couple of years but that was back in 2012 and I have not seen or heard anything on the subject since
 
OP
EricMark

EricMark

EF Member
Messages
20
Location
Mid Wales
As to measuring loop impedance if measured at DNO head and at central socket of a ring final, and subtract former from latter then you have impedance of local circuit, so ohms law will give volt drop.

As to micro generation, I am not saying the voltage tolerance was changed as a result, but the tapping often needs changing, the generator needs to cut out once the maximum volt is reached, so if set too high, the micro generators will end up for much of the time locked out, and there will be a complaint, so the DNO is being forced to drop voltage tapping as a result.
 

davesparks

Forum Mentor
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12,817
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guildford
As to measuring loop impedance if measured at DNO head and at central socket of a ring final, and subtract former from latter then you have impedance of local circuit, so ohms law will give volt drop.
You have the impedance of the local circuit with the effects of parallel paths included.

I can see that in theory this method will work, but since its applicable to an existing installation and not design I think its easier just to switch the normal loads on and measure the voltage directly.
 

UNG

Forum Mentor
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3,355
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Nr Wigan, Lancs
As to micro generation, I am not saying the voltage tolerance was changed as a result, but the tapping often needs changing, the generator needs to cut out once the maximum volt is reached, so if set too high, the micro generators will end up for much of the time locked out, and there will be a complaint, so the DNO is being forced to drop voltage tapping as a result.
I think you are a bit deluded if you think the DNO is being forced to alter tappings to suit micro generation

Pre EU directive 240v +/- 6% gives a voltage range of 225.6v to 254.4v
Post EU directive 230v - 6% + 10% gives a voltage range of 216.2v to 253v as you can see the upper voltage limit changed very little post EU directive

If the supply voltage falls within the limits detailed above the DNO certainly won't be forced to do anything and will do little or nothing unless equipment is being changed within it's normal engineering cycle

There is a bit more to the electricity network than you seem to understand and while micro generation can be a good thing it can also contribute to the local voltage problem you are highlighting if there is a significant amount of localised micro generation which is beyond the DNO's control
 
OP
EricMark

EricMark

EF Member
Messages
20
Location
Mid Wales
Forgive me for maybe being dumb at 7:30 in the morning, but where are you getting a current from for ohms law? I only see a resistance and a final voltage?
On the IET lectures it was stated that for a final ring, you calculate at 20 amp centre and 12 amp even spread, so design current for circuit Ib = 26 amp, although no hard and fast rule, that is how the 106 meters for a ring final is calculated.

This is also why the ring final works so well when compared with a 2.5 mm sq radial, as you still have that 20 amp at end of radial, so only 32 meters with 20 amp radial.

With other circuits the item it supplies will give current, so an immersion heater on a 16A radial will still be calculated at 13A rating for immersion heater. Cooker not so cut and dried, since load can be higher than the MCB size, could be size of MCB or size of appliance, or use diversity.

My question in early years was can it come back and bite us? Could some one measure the impedance of the circuit and claim it has always been too high, so as the installer or the author of the inspection report could we be asked to pay for the correction costs?

However once I built the software (using Java script) to convert loop impedance measured into volt drop, I realised the error of being +/- 0.02 ohms at both incomer and end of circuit means unless the volt drop was really excessive it would be near impossible to prove over the limit, and of course if asked to fit a socket to supply a cooker extractor for example the design current for circuit Ib is the size of the cooker extractor not 13A. So it would be near impossible to prove not to UK spec.
 

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