testing Ra

[daveygh]

hi all

i have a 315 amp 3phase TT supply head from edf into an outdoor enclosure and it is connected to our mccb (with earth leakage relay and shunt trip) this then goes up to the plant room to a 400amp busbar (via an isolator). my main earth bar is upstairs with the busbar (as EDF dont want any earths in the enclosure ) connected to 4 earth rods with a 240mm earth cable -my question is when doing my Ra test do i need to do it from the mccb downstairs and have a long lead to the incoming earth bar connection ( with all other bonds disconnected) or can i do it at the isolator upstairs where the incoming earth is?

hope this makes sense

 

[Chr!s]

Dave

Are you relying on the the earth leakage relay to activate the shunt trip?

And is the earth leakage/ shunt trip affording fault current protection or is this afforded by the mccbs?

 

[kme]

I would do it at the isolator upstairs; though the results will be comparable, wherever you do it......

The comparatively low resistance of the phase wiring will make a negligable difference to the reading of the rods (for the same reason, there is no point in "nulling" normal test leads on a TT install - when measuring 50 ohms +, 0.2 ohms isn`t going to make a difference.

HTH

KME

 

[ADS]

Are you not testing Ze rather than Ra?

 

[daveygh]

DaveAre you relying on the the earth leakage relay to activate the shunt trip?

And is the earth leakage/ shunt trip affording fault current protection or is this afforded by the mccbs?

hi this is the setup we have

http://mail.aol.com/32761-111/aol-1/en-gb/mail/get-attachment.aspx?uid=1.32165763&folder=OldMail&partId=4&saveAs=ELR__CTs_Product_Aid_Final.pdf

its basically to protect the switchgear and associated cables to the Db's

all circuits upstream are protected by rcbo's or rcd's

 

[daveygh]

Are you not testing Ze rather than Ra?

there is no Ze on a TT system is there? its called Ra i thought

 

[Andy™]

there is no Ze on a TT system is there? its called Ra i thought

Ze is external loop.... i.e Ra

 

[ADS]

there is no Ze on a TT system is there? its called Ra i thought

I've heard this said before, but I disagree.

The definition of Ra is given as - the sum of the resistances of the earth electrode and the protective conductor(s) - it doesn't include the Line conductor.

Ze, on the other hand, is the earth fault loop

I haven't got the regs to hand, but I think it's page 50, the Max Zs tables for TT, - there's a note that says something like....if Ra can't be determined then Ze value can be used (or words to that effect.). - so there must be a Ze on a TT system.

 

[daveygh]

so what is the testing procedure for Ra? :C

 

[steptoe]

you have it spot on for testing Ra,

testing Ze on a TT system requires very specialised test gear, and involves driving numerous rods into the ground and measuring resistances between them and star point.

 

[ADS]

you have it spot on for testing Ra,testing Ze on a TT system requires very specialised test gear, and involves driving numerous rods into the ground and measuring resistances between them and star point.

Wrong way round, I'm afraid, Steptoe

 

[ADS]

so what is the testing procedure for Ra? :C

Using an Earth Electrode Resistance Tester - It's a dead test and detailed in GN3 - tests the resistance of the rod/s

I think if you then add it to the R2 figure for the CPC of the circuit, this gives you the correct Ra.

For example, think of a metal cooker.

This is obviously earthed.

To measure Ra for the cooker circuit, you would measure the R2 resistance from the cooker casing (Exposed Conductive Part), back to the MET.

You would then measure the resistance of the Earthing Conductor and the Rod, using an Earth Electrode Resistance Tester.

Adding the two measurements together gives you the Ra for the cooker cicuit.

This can then be compared to the Maximum Allowed figures in the regs.

Which is why they say if you don't know Ra, use Zs - as there wouldn't be much in it.

Remember, Ra is given as:

The sum of the resistances of the earth electrode and the protective conductor(s) connecting it to the exposed conductive part - GN3

 

[steptoe]

you are of course correct ADS,

Im just getting so fed up with all this earthing stuff Im starting to talk out of my 4R5E about it sometimes.

 

[daveygh]

thats ok then, we had a company come in to put in the earth rods and they done the earth rod resistance so i only need to do the r2 on the cable to the earth bar and add it on :Applaud

 

[Chr!s]

thats ok then, we had a company come in to put in the earth rods and they done the earth rod resistance so i only need to do the r2 on the cable to the earth bar and add it on

I wouldn't rely on some one else's work, do the tests.

If fault protection is afforded via mcbs then we need to use a earth electrode tester as the resistance Ra is critical(Test method 1 GN 3).

If an Rcd is affording protection then we can use a earth loop impedance tester to calculate Ra as high precision is not required.(Test method 2 GN 3), this reading is taken as the electrode resistance.

Reg 411.5.3

This reg is met if the earth loop impedance of the final circuit meets table 41.5

So where Ra is not known,it may be replace by zs, because if the zs is within table 45.1 then we are allowed to assume compliance with reg 411.5.3.

Dave i see no issues with testing at the isolator, though it will have to be a no trip test due to the earth leakage/shunt trip, id prefer to test on a high setting.

 

[ADS]

+1

 

[la poste]

I have a question.

If you measure Zs with an earth fault loop impedance tester plugged into a socket of a system with a TT earth rod and your electronic hand held meter says 18 ohms is this a correct reading of Zs?

If I plug my kewtech Loop tester into a socket on a TT domestic system with no TNCS and my meter says !8 ohms is this a correct figure?

Can I rely on this figure as an accurate measurement of the resistance of my earth fault current?

Cheers.

 

[Andy™]

I have a question.If you measure Zs with an earth fault loop impedance tester plugged into a socket of a system with a TT earth rod and your electronic hand held meter says 18 ohms is this a correct reading of Zs?

If I plug my kewtech Loop tester into a socket on a TT domestic system with no TNCS and my meter says !8 ohms is this a correct figure?

Can I rely on this figure as an accurate measurement of the resistance of my earth fault current?

Cheers.

Zs on a socket will not give you PFC. you would need PEFC & PSCC from DB/service head

 

[steptoe]

I have a question.If you measure Zs with an earth fault loop impedance tester plugged into a socket of a system with a TT earth rod and your electronic hand held meter says 18 ohms is this a correct reading of Zs?

If I plug my kewtech Loop tester into a socket on a TT domestic system with no TNCS and my meter says !8 ohms is this a correct figure?

Can I rely on this figure as an accurate measurement of the resistance of my earth fault current?

Cheers.

ray

headbang

 

[la poste]

Ok then here's another daft question.

When I measure loop impedance of a socket on a TT installation is the reading I get the resistance of the current path from my tester through the cpc of the socket, through the cpc of the domestic ring circuit to the consumer unit then down through the earth via the earth rod and metal bonded pipework and onward through the general mass of the earth to the supply transformers earth, then back along the phase from the transformer to the consumer unit in the house and back along the phase of my ring circuit to the live red probe on my tester.

Is this a fair assessment of the path of my earth fault current?

Is this the circuit from start to finish?

And is 18 ohms the figure for the resistance of this circuit?

Cheers.