Voltage drop in building networks

I think this is another of those things that falls through the cracks of the BNO model.

The DNO responsibility ends at the incomer. All the rest is a BS7671 domain, and ideally the installation in the top flat should have the resistance of the riser/lateral that supplies it factored into its design, much as you would in a large factory with many boards, or  sub-mains to outbuildings or whatever.

I very much doubt however, if this is done rigourously, and I suspect a lot of top floor flats get a double dose of voltage drop and Zs. The saving grace is diversity - the breakers in the flat add up to 150A, but the riser can be assumed to carry no more than 50 ?  well if it is 16mm cable running bunched with a load of others through insulation, it needs to be.

As far as I know, the ESQCR limits apply at the outgoing meter terminals (or outgoing isolator terminals where the isolator is provided by the Meter Operator) regardless of whether there is a BNO between the intake and the meter. 


Regards,


Alan.

Having looked at the ESQCR, I agree that the voltage requirement must be met at the junction between the supplier's tackle and the consumer's, i.e. the load side of any meter.

I suspect that in most cases, that this is a non issue. Most building networks seem to feed relatively small premises such as small blocks of flats or a row of small lock up shops. In such circumstances it entails little extra cost, if indeed any cost, to keep the voltage drop in the building network to under 2%. In a small shop or flat, lighting circuits will often have only 1%voltage drop in any case, and 3% for power circuits.


In the case of a larger or more complex building network, then IMO engineering judgement may be applied to justify a greater voltage drop. A large building network implies either an on site substation, or at least a dedicated cable from a nearby substation.

In such circumstances it would be reasonable to design "as if" part of the installation belonged to the DNO, it would have done in the recent past. DNOs often allow for at least 5%voltage drop in street mains, and another 2% in service cables.

So it might be reasonable to allow for 5% drop in the street mains or in that part of the building network that corresponds to the street mains. And another 2% voltage drop in the service cables, or that part of the building network that corresponds to the service cables.

And then the usual voltage drop within the installation of the final consumer.

Consider the following example, firstly a row of homes and shops supplied directly by the DNO.


Voltage at transformer, 250 volts.

Voltage at far end of longest street main at full load, 235 volts.

Voltage at far end of "worst" service cable, 230 volts.

Voltage at far end of "worst" sub circuit in most distant consumers installation, 220 volts.


Remembering that those are worst case figures and that most customers will get about 240 volts. Entirely normal.


Now consider a similar installation but in which much of equipment belongs to a building network and not the DNO, and with similar voltages.

Most loads get about 240 volts, and the very worst example at full load still gets 220 volts.

By normal standards the voltage drop may seem excessive, but the actual voltage at the load is no worse than in the DNO example.

Exactly, and in these cases you must remember that an ESI spec transformer still has a secondary voltage of 433V at nominal tap. 


Regards,


Alan.

As far as I know, the ESQCR limits apply at the outgoing meter terminals (or outgoing isolator terminals where the isolator is provided by the Meter Operator) regardless of whether there is a BNO between the intake and the meter. 

Thank-you Alan, that's a very useful point.


I take the point that by starting with a higher voltage at the transformer the distributors have a larger margin to play with - but I daresay there are a few situations where they've still used it up.


Paperwork wise, I notice the ESQCR allows the option of agreeing in writing a different limit with the customer - so I guess it would be acceptable option for the BNO's v.d. to be added onto the DNO's (even if that took it over the normal 6% limit) - effectively obliging the customer's designer to work to tighter than normal limits for the customer's installation.


   thanks all

      - Andy.

EV charging equipment tolerant only to +/- 10 % of the stated rated voltage range

While the previously 240V areas like us moved to 230V-6%+10% I suspect the previously 220V areas (i.e. continental Europe) are now on 230V-10%+6% (I think they gave up on a common +10%-10% tolerance for all of us a while ago). So it sounds like someone's just forgotten installation v.d. again.


On the other hand, if EV charge points did start randomly dropping out when the voltage got towards the lower limit, that might not be an entirely bad thing from a grid stability and/or local network loading point of view....


    - Andy.