Apologies if this is a bit long. I’d be grateful if you folks could help me understand what (if anything) is normally done (and how they interpret the regs) as regards measuring EFLI in final circuits, particular in TT installations (in which case I actually wonder if there is any point in measuring {rather than calculating} them) .....
Whilst there is some pragmatic merit in testing an installation ‘as is’, the regs do not allow one to rely upon water/gas service supply pipes as a means of earthing. It can therefore be argued that, as far as the regs are concerned, one really should demonstrate that EFLIs are (would be) adequately low even in the absence of earthing via such routes.
Whilst I have seen some debate about the disconnection of MPB for the purpose of EFLI measurement, I don’t think that’s what most people do (could be regarded as dangerous) and it’s certainly not what GN3 says. In any event, most installations have multiple other ‘incidental’ parallel paths to earth via the supply pipes, so removal of MPB alone does not necessarily achieve much. As a result of this, EFLI measured at the level of final circuits may well give figures that largely reflect the service pipe earthing, particularly with TT installations.
Consider my (TT) installation. Ze of the earth rod alone is usually around 75-80 Ω (a bit higher when weather is very dry). However, EFLI measured anywhere in the (many) final circuits is invariably under 1 Ω, considerably under in many cases. This is explained by the fact that the measured ‘Ze’ of the MPB alone is about 0.45 Ω and, even with earth rod and MPB disconnected, ‘Ze’ measured at the MET (i.e. reflecting ‘parallel paths’ to earth via the supply pipework) is still only about 0.55 Ω – i.e. the MPB and ‘parallel paths together’ give an effective Ze under normal operation of about 0.25 Ω.
If one is content with measurements of final circuit EFLIs with the installation ‘as is’ (with supply pipes providing the main path to earth), then those measured EFLIs are obviously fine. However, if one interprets the regs as requiring EFLIs to be acceptably low even if all routes to earth via supply pipes were to disappear, then those measured figures are obviously ridiculously low.
Hence I wonder what people do. Do they content themselves with the measured EFLIs, or do they just measure Ze and then calculate Zs as Ze+R1+R2? Although the difference is only going to be dramatic with TT installations, there will still probably be an appreciable difference with many TN systems.
Kind Regards, John.
Whilst there is some pragmatic merit in testing an installation ‘as is’, the regs do not allow one to rely upon water/gas service supply pipes as a means of earthing. It can therefore be argued that, as far as the regs are concerned, one really should demonstrate that EFLIs are (would be) adequately low even in the absence of earthing via such routes.
Whilst I have seen some debate about the disconnection of MPB for the purpose of EFLI measurement, I don’t think that’s what most people do (could be regarded as dangerous) and it’s certainly not what GN3 says. In any event, most installations have multiple other ‘incidental’ parallel paths to earth via the supply pipes, so removal of MPB alone does not necessarily achieve much. As a result of this, EFLI measured at the level of final circuits may well give figures that largely reflect the service pipe earthing, particularly with TT installations.
Consider my (TT) installation. Ze of the earth rod alone is usually around 75-80 Ω (a bit higher when weather is very dry). However, EFLI measured anywhere in the (many) final circuits is invariably under 1 Ω, considerably under in many cases. This is explained by the fact that the measured ‘Ze’ of the MPB alone is about 0.45 Ω and, even with earth rod and MPB disconnected, ‘Ze’ measured at the MET (i.e. reflecting ‘parallel paths’ to earth via the supply pipework) is still only about 0.55 Ω – i.e. the MPB and ‘parallel paths together’ give an effective Ze under normal operation of about 0.25 Ω.
If one is content with measurements of final circuit EFLIs with the installation ‘as is’ (with supply pipes providing the main path to earth), then those measured EFLIs are obviously fine. However, if one interprets the regs as requiring EFLIs to be acceptably low even if all routes to earth via supply pipes were to disappear, then those measured figures are obviously ridiculously low.
Hence I wonder what people do. Do they content themselves with the measured EFLIs, or do they just measure Ze and then calculate Zs as Ze+R1+R2? Although the difference is only going to be dramatic with TT installations, there will still probably be an appreciable difference with many TN systems.
Kind Regards, John.
