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 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