TT Ra assistance

[Amp David]

Evening poeple. Could one of you fine chaps tell me where i'm going wrong please

I measured an Ra of 98.3ohms on a install, so worked this out to be 2.34amps PEFC. When I use my meter to fing the PEFC, its giving a reading of 0.64kA.

Is it me being stupid and totally missing something here

 

[pushrod]

parallel earths through the protective bonding to gas and water?

 

[Amp David]

No services in the garage.

Garage is TT and the CPC in the supply cable hasn't been connected to the garage CU but insulated.

 

[pushrod]

meter was just giving you a pfc and reading the pscc?

 

[Amp David]

Only using 2 leads, green clipped to the earth bar and the red probe on the incoming line on the main switch.

 

[GStueyXR]

Spooky....

Deffo no bonding? or cross connection with neutral anywhere?

Cant even imagine how you would get such a result !

 

[pushrod]

It had just started lashing down with rain improving the Ra - ok i give up, at least i tried

 

[Amp David]

Spooky....

Deffo no bonding? or cross connection with neutral anywhere?

Cant even imagine how you would get such a result !

No bonding anywhere. Main switch was open during test so if there was a cross connection it shouldn't have made a difference.

I'm baffled. Will still use the worked figure for the Ra, just wish I hadn't checked it with me meter now. Paranoid

 

[wallyanker]

Evening poeple. Could one of you fine chaps tell me where i'm going wrong please

I measured an Ra of 98.3ohms on a install, so worked this out to be 2.34amps PEFC. When I use my meter to fing the PEFC, its giving a reading of 0.64kA.

Is it me being stupid and totally missing something here

Sorry to sound really dumb but what does Ra stand for?? Is it the same as Ze but called Ra on a TT system??

Thanks

 

[GStueyXR]

Sorry to sound really dumb but what does Ra stand for?? Is it the same as Ze but called Ra on a TT system??

Thanks

Ra is the resistance at any point in a circuit where an earth fault may occur

 

[GStueyXR]

When you say no bonding does that include supplementary bonding too?

Maybe there is supp in bathroom connected into the CPC ????

 

[GStueyXR]

But then, I suppose you must have disconnected the the main earth and are testing purely the earth electrode....

Please investigate.... I need to know

 

[wallyanker]

Ra is the resistance at any point in a circuit where an earth fault may occur

Thank you

So for working out PFC, nominal voltage divide by Ze or Zs = 230 / 98.3 = 2.34

Is it possible the meter is at fault if the reading is that far out?

 

[Amp David]

But then, I suppose you must have disconnected the the main earth and are testing purely the earth electrode....

Please investigate.... I need to know

Main earth conductor disconnected from earth bar, then testing between that and the incoming line on the main switch,(main switch open of course)

I will be finding what caused it as its stuck now in the back of my mind and will be for ever i think.

Test meters fine, as using it all day every day and all results are fine.

Will be back there friday anyway so will test again. Wouldn't mind, but the Megger is so simple to use so its not as though i've messed up setting it up

 

[pushrod]

Sorry to sound really dumb but what does Ra stand for?? Is it the same as Ze but called Ra on a TT system??

Thanks

Basically yes - it is usually defined as the total resistance of the electrode/rod and the protective conductor connecting it to exposed conductive parts, but obviously it is the earth electrode that has the most significant contribution to it.