Voltage drop

[pnewton]

I am not an electrician and wondered if anyone can explain why voltage drop is an issue in the following case:-
A 50mm sq two core cable (earth rods at each end), 500m long from the remote meter box to my Farm supplying 230Vac single phase.
If all my equiptment will work fine at 220Vac (and considering the supply has a +/- voltage tolerance that could realistically go down to that), then what is the real issue and why could it be a problem?

 

[flameport]

The problem with voltage drop is that at the origin (where the meter is), the voltage can be in the range 216 - 253. (230V, -6% or +10%).
While equipment should function correctly in that range, excessively long cables in the installation will cause the voltage to drop even further.

If for example, the volt drop within the installation was 20V, this could mean that the voltage at the end was as low as 196V.
Items designed for 230 or 240V are not likely to work well, or at all at 196V.
Lights will be very dim, motors might not run properly or even start at all. Heating appliances will operate at a lower temperature and therefore take a lot longer to heat the water/air etc.
In extreme cases, the current which would flow in the event of a fault might not be enough to cause the circuit breaker or fuse to operate.

 

[ericmark]

Assuming 100 amp fuse it will take around 2 seconds for it to blow with a short circuit and you will have a 40 volt volt drop. And when you get near maximum load 4Kw of power is heating the ground. Someone is paying for that! Who depends where the meter is and what type of meter. The old ones worked out power used based on 230V so you would pay.
But the alternative is to get the supply authority to extend their high voltage cables and provide a transformer close to point of usage which they are not likely to do for nothing.
So why do you think it is an issue?

 

[Spark123]

A 50mm 2c swa has a resistive volt drop of 0.93 mV/A/m. (millivolts per amp per metre)
Assuming you have lighting the maximum volt drop between the origin and the current using equipment is 3% of the supply voltage, i.e. 3% of 230v = 6.9v.
Ignoring the volt drop for final circuits, which you will need to take into consideration;
Re arrange the equation for volt drop VD = (mV/A/m x A x m )/1000 to give the current;
(1000 x 6.9) / (0.93 x 500) = 14.83 amps

So overall, on volt drop your main cable can carry just shy of 15A if all of the lighting loading is directly at the other end of it.

 

[ban-all-sheds]

And if we assume a 100A loading (OK - maybe unrealistic, but it's one possible bound, i.e. the service fuse), VD would be 46.5V....

Why is there an earth rod at each end? TT supply or TN-C-S? Is adding 0.4Ω to the fault loop going to cause problems?

 

[ericmark]

Because of 705.411.4 most farms are on TT supplies. Yes I know you can have TN-S but it is rare.
As to actual volt drop on 50mm cable there are assorts of correction factors so figures given are only rough.
But main thing is you are unlikely to correct it will need the supply authority to provide another transformer and the extra costs for a second supply may not be worth it if you have no problems to date.

It is easy to say it is wrong but no so easy to find a cure.

 

[ban-all-sheds]

705.411.4 says no TN-C, not no TN-C-S...

 

[ericmark]

I agree and same with caravans and boats but seems many have got it into their heads that it has to be TT. And since many farms are also supplied from transformer on a pole it is often TT anyway.

But although I have used step up to 3.3KV and then stepped down again I would not think he really has that option and to use cable bigger than 50mm will also be high cost as will second supply transformer.

So I can't see easy way out. And although I can see the problems with volt drop if it's not causing a problem is there any point in spending the sort of cash that will be needed?

It is one of those problems where one would need to go there and see the problem for once self before giving advice as to what to do.

What I can't understand is how did it get that way to start with?

 

[Spark123]

Re the TN-CS and farms, there are certain places on a farm where it may be preferable not to use a TN-CS system. Milking parlours is one that springs to mind as and small voltages in the system may cause the cattle discomfort. Extra precautions with TN-CS may be applied such as a bonded metallic grid in the ground.
You have to be carefull with TT systems too, not to allow livestock to within the area of pool of voltage created by an the earth electrode.

I agree and same with caravans and boats but seems many have got it into their heads that it has to be TT. And since many farms are also supplied from transformer on a pole it is often TT anyway.

If the transformer is there then there is no reason not to provide a TN system.

 

[pnewton]

Cheers for all your advice on this Guys. It is not that we have experienced any problems at all (in fact the voltage at the Farm was last checked as 244Vac, pulling close to 10A) and I accept that the voltage drop down the cable is an unfortunate energy 'loss' to the ground.

I am mainly concerned that if I have to have the electrical system tested at some point (alterations, building regulations and new part P etc etc) then it would not pass and get a certificate?

Is there anything that I could do to get the system to pass (that is financially viable)?

 

[ericmark]

The only mod which may be needed is to include RCD's but if it were me I would just see what is picked up on the report and then act on that.

 

[Spark123]

RCDs to overcome volt drop
It depends as to what you connect to the cable, if anything draws more than 14.8A then it will be out of spec on % volt drop.
Saying that, BS7671 isn't a statutory document so there is nothing forcing you to comply with it. As long as it isn't dangerous is the main thing.

 

[ericmark]

I was not thinking RCD would help on volt drop but also the Zs is likely to be on high side and RCD was to ensure with a fault it would trip.
However there is a question on if active RCD should be used? I would not use an active type but strictly speaking I suppose you should as can't guarantee enough voltage to operate a passive type.

 

[Spark123]

Unless you're getting into some electronic RCDs which require a supply voltage to work, RCDs operate on current.

 

[ericmark]

Most RCD's when opened look something like

and have some electronic components. Look by the number 7 that looks like a semi-conductor device. Yes some old ones may be without electronics but most do have some. If you look

Sorry link does not work try here you will see this one does actually state the voltage it will work at on the device. In most cases one has to do a lot of work to find the lower voltage limit.
Because we had problems when working on Sizewell Power Station where someone had fitted 230 volt RCD's on the 110 volt system and they failed to work I know even back in 1990 RCD's did require a certain voltage to work. Which is over the 63.5 volt available on a 110 volt transformer to earth.
However because of the problems with brown out RCD failure and de-frosting personally I would take a chance and fit passive types rather than the active type. The whole idea of the active type is if one has a volt drop they fail safe there would be no reason to produce them if the passive type did not have min operation voltage.
I have see quoted a very low voltage as the standard to which they must now operate on but I have never seen any official document giving these figures so on a caravan site for example I would fit the active type.