BS7671 switched protective conductors

Hi Bill,


I think the real answer to your question is "It depends...."

You have to think about what you are trying to achieve technically rather than just blindly following the words of the clause. You want the system to be earthed for safety.

If you disconnect one earth and prior to connecting the other earth connection an earth fault is introduced onto the system, when you close the second earth switch you will be making onto a fault and the switch will have to be rated to be suitable to close onto that fault current. Having said that, the chances of an earth fault being introduced onto the system are very remote as it would require additional switching operations to occur between disconnection of one earth switch and connection of the second.

Alternatively if you make before break, the issues are likely to be circulating currents between generators through the earth connections, possibly at third or higher orders of harmonics. If the circulating currents are likely to be low then it would not matter if there was a brief period between the make and break but if they are likely to cause protection problems then you would not want to do it (but also you would probably want to look at the setup of the generators and the voltage control).

My feeling is that in 99.9% of cases (possibly higher) it will not really make a difference, but look at how you are intending to operate the system and make the decision on that basis.

The other thing to consider is does the stand-by generator really need an independent means of earthing? You may not need to do anything if the earthing is at the local switchboard.


Alasdair Anderson

Hi Alasdair, not quite the situation I was considering, basically, I'd prefer to ensure that the earth and neutral connections are in place prior to closing the live condutor circuit. My assumption it that the switching would always pass through an "Off" state. the situation to be avoided being live pole(s) closing before the earth/neutral circuits are in place. The wording I quoted doesn't I feel make this a requirement.

BTW I agree completely, we're meant to be engineers thinking about what we're doing no just blindly following the text.

Bill

Bill,

I think you really need to look at why this is required in the greater picture.

With the DNO network being primarily TN-C, that is with CNE or PEN conductors, and being largely in a very poor state of repair, the requiement is there to limit the possibility of diverted and stray currents for one thing.

Also with Neutral being a live current carrying conductor, one needs to be careful of the switching arrangements of that also.


HTH

My guess is that they have assumed that for standard commercial 3 or 5-pole rotary switches, the time difference between the different contacts connecting is negligible.  So it's unlikely that anybody would be electrocuted in the fraction of a second between the live connecting and the earth.


If you wanted to guarantee that the earth connected before the live, you'd have to have a special switch made to do that.