Earth rod value ??

[Matt Thornton]

I tested a TT system today earth rod impedance was 1467 ohms, I say thats a pass ie, less than 1667 ohms, the QS at work says that it's a fail, it must be less than 200ohms......who's right ??? and why ??

 

[Mac]

Regulation 413-02-20 states that, for TT systems, the
value of the earth electrode resistance, R​

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A (Ω),
multiplied by the operating current of the protective
device, In (A), must not exceed 50 V. Therefore, in the
event of a fault to earth, the RCD must operate to
prevent a danger of shock from exposed metalwork.
The resistance of the earth electrode should be less
than 50/In Ω; with an RCD rated at 30 mA, this
equates to approximately 1666 Ω.
Using this example, theoretically, it can be shown
that a suitably rated RCD will permit high values of
RA, and therefore of Zs, than could be expected by
using the overcurrent devices for indirect contact
shock protection. Guidance Note 3 advises that in
practice, however, measured loop impedance values​

above 200 Ω will require further investigation.

 

[Matt Thornton]

Sorry, forgot to say that main switch is a 100mA time delayed RCD

 

[Guest123]

I tested a TT system today earth rod impedance was 1467 ohms, I say thats a pass ie, less than 1667 ohms

Honestly amazed at that statement.

To the current edition of BS 7671 the MAX Ra (earth electrode) value is 200 ohms.

Some scheme providers say even less i.e NICEIC - 100 ohms.

With your 100mA RCD the MAX Ra you are permitted to for it to operate before fault current exceeds the safe touch voltage of 50V is 500 ohms 1667 ohms only applies when a 30mA RCD is present, which is still over the 200 ohms specified in ol'red.

So your QS is bang on...and you have a problem.

 

[Monkey1984]

Honestly amazed at that statement.

To the current edition of BS 7671 the MAX Ra (earth electrode) value is 200 ohms.

Some scheme providers say even less i.e NICEIC - 100 ohms.

With your 100mA RCD the MAX Ra you are permitted to for it to operate before fault current exceeds the safe touch voltage of 50V is 500 ohms 1667 ohms only applies when a 30mA RCD is present, which is still over the 200 ohms specified in ol'red.

So your QS is bang on...and you have a problem.

Pretty certain there is a table for this in the Regs and the OSG as well. Maybe someone can inform the OP what page or Regulation this falls under. (I would do but that would involve walking to the van and I'm a lazy git!)

But as mentioned the Maximum permitted is 200Ohms plus the resistance of the Rod (21 Ohms??), The 100ma RCD does equate to 500Ohms as well. So your value fails.

I suggest trying a different location for the Rod and maybe a copy of the OSG to keep hold of for the future as well.

 

[Des 56]

Pretty certain there is a table for this in the Regs

The op should look at page 50 of the big red book table 41.5

 

[Steve1412]

Dont think the 'quick fix' solution will work on this one...even after 8 pints of stella..

 

[Monkey1984]

Dont think the 'quick fix' solution will work on this one...even after 8 pints of stella..

Imagine the look on the customers face.