Mike M:

Andy thanks for the response.


BS7671:2018 states


"Voltage, nominal. Voltage by which an installation (or part of an installation) is designated. The following ranges

of nominal voltage (rms values for AC) are defined:"

– Extra-low. Not exceeding 50 V AC or 120 V ripple-free DC, whether between conductors or to Earth.

– Low. Exceeding extra-low voltage but not exceeding 1000 V AC or 1500 V DC between conductors, or

600 V AC or 900 V DC between conductors and Earth.

– High. Normally exceeding low voltage.


For the protective conductor not to be categorized into one of these groups it would mean that the protective conductor is not part of a system that has a designated nominated voltage. If that is the case then I agree however I am unable to find something to back this up in BS7671, perhaps this is further defined somewhere else ?

Mike M:


For single core green/yellow insulated equipotential bonding cables lets say originating from an earth bar and bonding some structural steel work, are these classed as FELV under BS7671:2018 ?

Andy thanks for the response.


BS7671:2018 states


"Voltage, nominal. Voltage by which an installation (or part of an installation) is designated. The following ranges

of nominal voltage (rms values for AC) are defined:"

– Extra-low. Not exceeding 50 V AC or 120 V ripple-free DC, whether between conductors or to Earth.

– Low. Exceeding extra-low voltage but not exceeding 1000 V AC or 1500 V DC between conductors, or

600 V AC or 900 V DC between conductors and Earth.

– High. Normally exceeding low voltage.


For the protective conductor not to be categorized into one of these groups it would mean that the protective conductor is not part of a system that has a designated nominated voltage. If that is the case then I agree however I am unable to find something to back this up in BS7671, perhaps this is further defined somewhere else ?

Mike M:

Guys,


BS7671:2018 is classing cables extra low voltage (ELV) as an operating voltage not exceeding 50Vac or 120V ripple-free dc.


BS7671 goes on to further categorize ELV cables into SELV, PELV, FELV.

Sparkingchip:

Mike M:

Guys,


BS7671:2018 is classing cables extra low voltage (ELV) as an operating voltage not exceeding 50Vac or 120V ripple-free dc.


BS7671 goes on to further categorize ELV cables into SELV, PELV, FELV.

I am not sure you have grasped the distinction between the three categories.


The main distinguishing feature is the quality of the insulation used in the transformer and connected equipment. The note to 411.7.1 describes FELV as insufficiently insulated with respect to circuits at higher voltage.


The earthing of ELV equipment is to make up for weaknesses in basic insulation, good quality SELV equipment is sufficiently insulated from circuits at higher voltages and earth.


Now what was your question?


 Andy Betteridge 

 

AJJewsbury:

Andy thanks for the response.


BS7671:2018 states


"Voltage, nominal. Voltage by which an installation (or part of an installation) is designated. The following ranges

of nominal voltage (rms values for AC) are defined:"

– Extra-low. Not exceeding 50 V AC or 120 V ripple-free DC, whether between conductors or to Earth.

– Low. Exceeding extra-low voltage but not exceeding 1000 V AC or 1500 V DC between conductors, or

600 V AC or 900 V DC between conductors and Earth.

– High. Normally exceeding low voltage.


For the protective conductor not to be categorized into one of these groups it would mean that the protective conductor is not part of a system that has a designated nominated voltage. If that is the case then I agree however I am unable to find something to back this up in BS7671, perhaps this is further defined somewhere else ?

Sometimes you have to work things out from what isn't said.


Firstly protective conductors don't have a nominal voltage - in most systems (e.g. ADS) they carry the potential to which the system is nominally referenced (typically "Earth"). If you do want to consider the actual (rather than nominal) voltages that a protective conductor might carry (above true Earth say) then you'll find that it's not particularly related to the circuit's/system's nominal voltage either as potentials may be imported from other circuits, or other installations or even HV faults from the distribution system - so that doesn't help either.


Then consider why you'd want to allocate them to a voltage band - normal reasons would be for rating of functional insulation and acceptable approaches for shock protection - neither of which generally apply to protective conductors as they have no requirement to be insulated either for functional or safety reasons (the requirement for smaller protective conductors to sheathed with the equivalent of basic insulation is purely for mechanical and corrosion protection).


   - Andy,

 

Mike M:

I am not sure you have grasped my question. SELV,PELV,FELV are already very well defined in BS7671:2018. My question was BS7671:2018 is classing cables extra low voltage (ELV) as an operating voltage not exceeding 50Vac or 120V ripple-free dc, For single core green/yellow insulated equipotential bonding cables lets say originating from an earth bar and bonding some structural steel work, are these classed as FELV under BS7671:2018 ?


If you read BS7671:2018 you will see that FELV does not need to meet the isolation requirements imposed on SELV and PELV.

 

gkenyon:

Mike M:

I am not sure you have grasped my question. SELV,PELV,FELV are already very well defined in BS7671:2018. My question was BS7671:2018 is classing cables extra low voltage (ELV) as an operating voltage not exceeding 50Vac or 120V ripple-free dc, For single core green/yellow insulated equipotential bonding cables lets say originating from an earth bar and bonding some structural steel work, are these classed as FELV under BS7671:2018 ?


If you read BS7671:2018 you will see that FELV does not need to meet the isolation requirements imposed on SELV and PELV.

 

I think we've already answered this - they are generally classed as Low Voltage for an LV installation. When connected in an installation complying with BS 7671 and relevant legislation, they are generally considered safe to touch (give or take the debate around broken PME Neutrals).

 

Mike M:

The conclusion that we had arrived at was that was with nominal voltage being defined as voltage by which or part of an installation is designated we would have to say that the equipotential bonding cables do not form part of a system that has a designated voltage and therefore are not ELV or LV

1. For electrical and electronic system maintenance activities, protective conductors and functional earthing conductors may have to be treated like live LV conductors when they are disconnected at one end.

• Also common is the case where a screen, armour, etc., is connected to one earthing system at one end, and the other end is presented at part of an installation with a different earthing system. Again, we need to treat that protective conductor or functional earthing/bonding conductor in the case of a screen, at the very least in the same manner as a live conductor - gapping and insulating against contact.

gkenyon:

Mike M:

The conclusion that we had arrived at was that was with nominal voltage being defined as voltage by which or part of an installation is designated we would have to say that the equipotential bonding cables do not form part of a system that has a designated voltage and therefore are not ELV or LV

I don't agree with your conclusion under all circumstances.


If the cable is correctly connected at BOTH ends then it can definitely be classed as having Earth potential, and there's not always a need to even insulate it. Protective conductors do not always require insulation.


HOWEVER, if one end is disconnected for whatever reason, then a hazardous condition may be prevalent.


So, as I have previously intimated, it depends why you are asking the question. There are two cases in particular which must be highlighted:

1. For electrical and electronic system maintenance activities, protective conductors and functional earthing conductors may have to be treated like live LV conductors when they are disconnected at one end.

• Also common is the case where a screen, armour, etc., is connected to one earthing system at one end, and the other end is presented at part of an installation with a different earthing system. Again, we need to treat that protective conductor or functional earthing/bonding conductor in the case of a screen, at the very least in the same manner as a live conductor - gapping and insulating against contact.

I apologise for having to restate this point, but it's very important if someone is reading this thread because they are assessing risks for certain work activities.