Testing a TP & N board...

P

person

I've just started learning about testing and I know how to do the majority of the tests but was just wondering how you would do your Ze etc when theres no incoming earth on a TP & N board?
 
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Are you saying that the board has no earth connection.
 
Not sure I understand?

Why would a TPN board have no earth? Why would a TPN board have no earth any more than a SPN board?
 
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There should also be an incoming earth with a TT system!

Is he talking about a board not at source? In other words, Zs (or Zdb)?

Ze should always be measured at the intake with the installation isolated and all outgoing earthing disconnected.
 
I just got the impression that a TP & N board was three phase & neutral which would have a swa with 4-cores? or would an earth also be run in?
 
or would an earth also be run in?

Yes, there should be an earth run to the board. If the supply is via 4-core SWA cable, then the earth will be the cable armor. There might also be a separate earth conductor run in parallel.
 
Yes, armour can be used as an earthing conductor as long as it meets the required for size by calculation.
If not then you'll need to supliment it with a separate earth.
 
There was an article by John Peckham on the IET web site on using SWA as an earth and he points out how with larger cables (over 95mm) how the SWA may not be enough.
He lists the CSA required to meet with 54.7 and has a chart to show what does and what does not comply but I remember all too well finding certain glands would not fit certain cable because the SWA was too thick so I am sure different manufactures use different spec for the steel and although often marked with core sizes I have seen very little on cable as to size of steel wire armour. TABLE 54.1 gives 10mm as minimum size for steel earth and in John Peckham's chart 1.5mm SWA has 15mm of SWA but it does not state individual core sizes and with smaller cables even a pin prick of a hole in outer PVC would soon corrode cable and I know with annual testing I have seen 35mm cables very quickly lose their earth when damaged.
 

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