Ask The Trades - RCD's TT systems

oldspark GDPR opt-out	RCD's TT systems Sep 15^th, 2009, 8:36pm If my Zs impeadance is on the high side, will this affect the tolerances when using RCD's.
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wozzy Trade Member Offline Ask The Trades Posts: 584 Total Thanks: 4 For This Post: 0 Gender: Trade: Electrician	Re: RCD's TT systems Reply #1 - Sep 15^th, 2009, 9:25pm anything above 200 ohm is considered unstable, as far as your rcd goes the maximum i believe is 1667 ohm. which relates to the touch voltage calculation.
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dingbat Trade Member Offline How hard can it be? Posts: 2822 Total Thanks: 2 For This Post: 0 Gender: Trade: Electrician	Re: RCD's TT systems Reply #2 - Sep 16^th, 2009, 6:24am The maximum values of Zs given in table 41.5 on page 50 of BS 7671 are the maximum values to allow disconnection in the event of an earth fault on equipment. They are calculated by dividing 50V by the rating of the RCD, so (as wozzy has said) 1667 Ohms for a 30mA, 500 Ohms for a 100 mA and 167 Ohms for a 300 mA.
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howlingdog GDPR opt-out Total Thanks: 2 For This Post: 0	Re: RCD's TT systems Reply #3 - Sep 16^th, 2009, 9:08pm All above correct. I'd take an Ra reading at the mains end and make sure it's ok. Then a Zdb. That way you have a starting base. Then if the circuit readings are in the ball park then fine. But you need to verify at the origin first.
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dingbat Trade Member Offline How hard can it be? Posts: 2822 Total Thanks: 2 For This Post: 0 Gender: Trade: Electrician	Re: RCD's TT systems Reply #4 - Sep 16^th, 2009, 9:19pm Quote: I'd take an Ra reading at the mains end and make sure it's ok. Really? Sure you wouldn't use a loop tester, like everybody else? Quote: Then a Zdb. Why not just a Ze, like everybody else? The term Zdb normally refers to a Zs value taken at the end of a sub-main, with all bonding connected.
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howlingdog

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Re: RCD's TT systems
Reply #5 - Sep 17^th, 2009, 7:53pm

The Ra is the 'Ze' of the rod, if you like. On a TT there is no Ze it is called Ra. The Zdb (Zs of the board with all bonding connected) is taken to be a base for all Zs readings of the final sub-circuits. If the Zdb is say 5 Ohm and the final sub-circuit is say 15 Ohm then a problem will exit is the final sub-circuit. This will not be a prob for the rcd but will show that there is a prob with the circuit wiring.

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howlingdog GDPR opt-out Total Thanks: 2 For This Post: 0	Re: RCD's TT systems Reply #6 - Sep 17^th, 2009, 7:55pm An Ra value may be taken with a EFLI tester or 'the other thing'. either one will do.
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Lectrician

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Re: RCD's TT systems
Reply #7 - Sep 17^th, 2009, 9:16pm

With a TT system, many folk will take the Ze to be the reading to the rod (rod and the MEC). They also often record a Zdb (the reading at the board with the MEC connected and all bonding and CPC's connected). This is purely for reference and not required, but useful to know. This would be called a Zdb.

Ra is the resistance of the rod, and as far as I thought could be measured with your loop tester, or the meter that no one owns!

Should the Ra box on certs be left blank if you are using a loop tester Ding?

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Y3 GDPR opt-out Total Thanks: 109 For This Post: 0	Re: RCD's TT systems Reply #8 - Sep 17^th, 2009, 9:23pm I too thought Zdb was the term given for the EFLI being carried out at the incomer of a CU/Dist board, positioned at the end of a distribution circuit?
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Lectrician

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Re: RCD's TT systems
Reply #9 - Sep 17^th, 2009, 9:39pm

Quote:

I too thought Zdb was the term given for the EFLI being carried out at the incomer of a CU/Dist board, positioned at the end of a distribution circuit?

Or when you do not test to just the external earth......(the Ze). If you test the main DB without disconnecting the bonding or testing directly to the MEC, you are doing a Zdb.

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Y3

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Re: RCD's TT systems
Reply #10 - Sep 17^th, 2009, 9:46pm

Lectrician wrote on Sep 17^th, 2009, 9:39pm:

Quote:

I too thought Zdb was the term given for the EFLI being carried out at the incomer of a CU/Dist board, positioned at the end of a distribution circuit?

Or when you do not test to just the external earth......(the Ze). If you test the main DB without disconnecting the bonding or testing directly to the MEC, you are doing a Zdb.

Where is this printed then, ive never come across this with all the reading that i do? whats MEC, Main Earth Connection?

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Lectrician Administrator Trade Member Author Offline Ask The Trades Posts: 8815 Total Thanks: 109 For This Post: 0 Braunton, North Devon United Kingdom Gender: Trade: Electrician	Re: RCD's TT systems Reply #11 - Sep 17^th, 2009, 10:15pm It's not written anywhere, but a Ze is external. Leave anything connected and it is no longer a Ze. If you do a PIR and cannot isolate (this does happen), you record a Zdb at the origin, and omit dead tests.
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Y3

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Re: RCD's TT systems
Reply #12 - Sep 17^th, 2009, 11:28pm

Lectrician wrote on Sep 17^th, 2009, 10:15pm:

It's not written anywhere, but a Ze is external. Leave anything connected and it is no longer a Ze.

If you do a PIR and cannot isolate (this does happen), you record a Zdb at the origin, and omit dead tests.

Ok, thanx for that piss head, still able to function drunk? You are good!! Wink

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dingbat Trade Member Offline How hard can it be? Posts: 2822 Total Thanks: 2 For This Post: 0 Gender: Trade: Electrician	Re: RCD's TT systems Reply #13 - Sep 18^th, 2009, 4:12pm Quote: The Ra is the 'Ze' of the rod, if you like. On a TT there is no Ze it is called Ra. Not true. Refer to part of Lec's answer and also to the source of all correct terminology: BS 7671 (See page 50, for a good explanation)
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dingbat Trade Member Offline How hard can it be? Posts: 2822 Total Thanks: 2 For This Post: 0 Gender: Trade: Electrician	Re: RCD's TT systems Reply #14 - Sep 18^th, 2009, 4:19pm Lectrician wrote on Sep 17^th, 2009, 9:16pm: Should the Ra box on certs be left blank if you are using a loop tester Ding? There's no RA box if you use the Appendix 6 forms. If you are measuring Ze, then record it as Ze on page 2; if necessary, modify the form to fit, or include an explanatory note.
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howlingdog GDPR opt-out Total Thanks: 2 For This Post: 0	Re: RCD's TT systems Reply #15 - Sep 23^rd, 2009, 7:30pm Ra is the loop path from the rod, to the tx, back through the windings, cables and back to the 'fault' that is the loop tester. The reading is the Ra.
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Y3

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Re: RCD's TT systems
Reply #16 - Sep 23^rd, 2009, 9:57pm

Quote:

Ra is the loop path from the rod, to the tx, back through the windings, cables and back to the 'fault' that is the loop tester. The reading is the Ra.

No it's not Wink

Ra is the sum of the resistances of the earth electrode and the protective conductor(s) connecting it to the exposed conductive part.

Page 48 of GN3

Regards
Carl

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