TT systems

S

sparky40

I still haven't been able to get my tenner bet !!!!

I maintain it's 2.5mm and my mate is going for 10mm minimum.

We always would fit a 10mm, i don't know why, but I would like to know how I can find the calculations for determining the size of the conductor.

Please help.....
 
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Surely the point is to determine what is the max current that will flow in the conductor from the MET to the earth spike?
In a TT installation this will be 100mA as this is the current at which the incomer will trip. Its an RCD, aint it?

As this is (or should be :rolleyes: ) a Type S RCD, the conductor may have to carry a much higher current for up to half a second but a much smaller condctor is needed than that in a TN system where the tail should be at least 50% of the meter tail csa.

TTC
 
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here's what the regs say on earthing conductors (which are different to bonding conductors)

543-01-03
The cross sectional area where calculated shall not be less than
S=squareroot(I^2*t)/k
S is cross section area in mm^2
I is the maximum current that can flow through the protective device in amps
t is disconnection time for current I in seconds
k is 143 for sheathed conductor not in a cable.
I presume that we can use the rating and disconnection time for I and t? Please correct me if I'm wrong because I'm not to sure on this. If so then for a 100 mA RCD operating in 200ms this gives
0.0003 mm^2 :!: :!: :!:

However it must also comply with
543-01-01
If the earthing conductor is not in trunking, cable or an enclosure it must be at least 2.5mm^2 or 4mm^2 if not protected from mechanical damage.
If buried it must comply with 542-03-01.

I think the point is that such a small current will be passing through the earthing conductor for such a short time that damage to the wire is not an issue so we just have to ensure that the wire is large enough to not incease Ze too much and to be sensibley durable.[/quote]
 
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Big_Spark

Very Good Phil, but what about Regulation 542-03-01...table 54A and more importantly...the old stallwart..table 54G

All earthing conductors should be selected by using 54G, the adiabatic equation is a red herring.
 
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OSG: (admittedly yellow copy)

For TT systems protected by an RCD with an earth electrode resistance 1 Ohm or greater, the earthing conductor size need not exceed 2,5mm² if protected against corrosion by a sheath and if protected against mechanical damage; otherwise see table 10C of Appendix 10.


Jim...If table 54G were crucial here, wouldn't the OSG mention that reference need be made to it?
 
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philrosenberg said:
I presume that we can use the rating and disconnection time for I and t? Please correct me if I'm wrong because I'm not to sure on this. If so then for a 100 mA RCD operating in 200ms this gives
0.0003 mm^2 :!: :!: :!:

It would be the PEFC (as thats the current that will actually flow) and 40ms in that case, but you'd only do it on an RCD in a TT system and it would pass by a mile most likely (unless you get an electrode value thats *way* below the norm for a TT), otherwise you'd be applying it to a fuse or breaker (btw)
 
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Big_Spark

If you want an honest opinion, I think much of section 54 is a **** up. If you read it carefully there is reference to a Regulation that hasn't even been printed in the new book, but existed in the old one...and there is no reference to it's deletion from the regs as with others.

If you actually compare the older version with the current version, there are a number of contradictions and misprints...I'll bet the whole kit kaboodle is Fubar..

Damn Joke considering the importance of the subject matter!!
 
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RCDs dont go over 500mA (0.5 amps) usually ? so the max current passing down the earth is in theory half an amp
2.5mm2 can take over 20 amp

but..we use 10mm2 when we install earth rods because it helps keep the earth loop lower and looks better
 
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Theoretically the current flowing down the earth conductor can be the voltage of the supply divided by the resistance of the path for as long as it takes the RCD to operate, as adam says the PEFC.
 
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Reading my regs book this morning at work I can see there are 2 choices for selecting size of a TT earthing conductor:-
1)Use adiabatic equation, so to work this you need the operating time of protective device (RCD), to know the operating time you need the PEFC and thus the Zs (Zs also needed for fault current part of equation).In most cases the size of earthing conductor falls below 2.5mm², except for maybe the extreme of really low Zs combined with unusally slow operating time.So 2.5mm² is minimum possible if you have all your konstants and are looking to save money on cable size but not on time working out
OR
2)Use table 54G and use 16mm² (with 25mm² phase)
 
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Big_Spark said:
Very Good Phil, but what about Regulation 542-03-01...table 54A and more importantly...the old stallwart..table 54G

All earthing conductors should be selected by using 54G, the adiabatic equation is a red herring.

542-03-01 Just says to follow reg 543 and also gives csa for buried cable as I said above, so yes for buried cable you need to use 54A. I'll give you that one ;)
Reg 543-01-04 basically says, if you can't or don't want to calculate csa using the adiabatic equation then you can use table 54G instead.

- Sorry for all the reg quoting. When I was a wee lad I used to play with toy soldiers and generally the winner of the games was the player who could take advantage of rules loopholes the best :)

Adam and other posts since,
Cheers for that, so if Zs was 1 Ohms then I would be 240A. I thought I must have something wrong. :(
So sticking this in the equation and also using a type S rcd that trips in 500ms would give S at least 1.2mmsq

The end result as far as I can see, is that for Zs at least 1 Ohm, with an RCD, protected from corrosion and mechanical damage 2.5mmsq is fine (as per securespark's OSG quote :)), if no protection from mechanical damage 4mmsq needed and if buried see table 54A. If you don't want to or can't measure Zs then you have to use 54G

Thanks for all the input everyone, that's helped me immensely.
 

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