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Existing TNCS site with Fixed Generator Adding Circuits

Former Community Member
Former Community Member
Sorry for the length of this.


I am adding some lights and sockets to existing circuits in a SP board. And providing bonding to metal oil pipes coming into this workshop from underground.

They have a diesel SP generator supplying (installed in the last 5 years)  ‘essential’ SP Dis boards across the site feeding lights and basic power - in case of supply interruptions.

It seems that the existing TP boards had SP circuits removed and diverted into these newer SP boards - so some metal cable management systems are shared between the standard TP circuits and those fed via the change over SP boards. The SP boards are being supplied by a 3 position change change over switch. The positions are: Mains on, all off, generator on. The idea is that the three position switch is thrown to manually, isolating street supply and switching over to the generator which is then also manually started.


There was no data left at all about the system design, or generator.  Okay i can go back to the manufacturer and obtain info about the generator..  But where can i find more about the regs governing SP generators and their integration into fixed TNCS systems please?


These 3 position switches also isolate and switch over the earths. This also means isolation from local bonds to the building and the oil pipes.  For now i have instructed the client not to use the switches nor the generator.


My concern here is that in all areas there is always a TP Dis boards supplying TP and SP non essential circuits in the area. Isolating the earth in this way seems dangerous, especially as all the bonds are on the TP boards. We are also in a reasonably high lightning area, so if earth was isolated this could also impact any surge protection - not that there is any installed yet .


I have yet to check, but assuming the Generator is both staked at source (i have seen a think earth wire go into the ground), and that it is also deliberately or fortuitously linked back to the MET in the mains distribution shed.


I think the action need to be taken here is: 


Link out the earth connection in the Triple pole changeovers so earth is not switched.  Provide additional warning that it should not be remove, inside and outside the box.

Ensure continuity of earth between Mains and Generator back to the earth stake at the Generator, this will mean though that i have an Earth stake at the Generator position that is also connected to a TNCS system.

With the generator running, assuming all other circuit related tests have been completed and recorded:

Check Prospective fault current and ZS to ensure the protective devices still comply.

Provide adequate signage in relation to bonds between the two earth systems as well as notes about dual supply and isolation.


Really looking for some guidance here please.


Thanks


john




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    If the earthing is switched it needs to be ganged so that it cannot be interrupted when L or N are in use.

    Usually with a bit of cunning, just  switching L and N is enough and earthing is left permanently attached (!)  It is not wise to have "earthing " that may or may not be connected to other 'earthing' that is simultaneously accessible, as there may be a voltage between two metal objects that you could be straddling. There may be a good reason to switch the CPCs, but  the normal arrangement is to have a common earth if at all possible.


    If the genset supply is to be run when the street supply cable may be severed, you cannot rely on the company earth alone, but there is no reason not to have your own electrode (s) in parallel with the MET all the time.

    There should be a single solid N-E bond inside or near to the genset, in such a way that the neutral switching means it is NOT in circuit when the external mains is in use - the DNO also provide an N-E bond, and there really should only be one NE bond in use at a time.

    I suspect there will be more,  but does this make sense with what has been done first ?

    Mike.


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  • 0


    If the earthing is switched it needs to be ganged so that it cannot be interrupted when L or N are in use.

    Usually with a bit of cunning, just  switching L and N is enough and earthing is left permanently attached (!)  It is not wise to have "earthing " that may or may not be connected to other 'earthing' that is simultaneously accessible, as there may be a voltage between two metal objects that you could be straddling. There may be a good reason to switch the CPCs, but  the normal arrangement is to have a common earth if at all possible.


    If the genset supply is to be run when the street supply cable may be severed, you cannot rely on the company earth alone, but there is no reason not to have your own electrode (s) in parallel with the MET all the time.

    There should be a single solid N-E bond inside or near to the genset, in such a way that the neutral switching means it is NOT in circuit when the external mains is in use - the DNO also provide an N-E bond, and there really should only be one NE bond in use at a time.

    I suspect there will be more,  but does this make sense with what has been done first ?

    Mike.


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