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bs7671
Hi there I have another question for you guys.What is the maximum Zs values for different circuit breakers i.e type B,C,D also where would we find them and why the difference?
Discuss Zs values? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net
Hi there I have another question for you guys.What is the maximum Zs values for different circuit breakers i.e type B,C,D also where would we find them and why the difference?
Look in your namesake.
on site guide and GN3 have tables with the standard kA ratings of all protective devices manufactures will give more detailsI want to find a table or something so I can see the Ka ratings as per different device type.Or at least someone to explain it in simpleton terms
why are osg tables 80% of bs7671?
on the eic you would still enter the max zs allowed though?
on the eic you would still enter the max zs allowed though?
even so, still put in the measured Zs in it's column, but max. as 1667.
That's exactly right, many people incorrectly assume this is just for TT systems.
1667 is for TT systems.
On a TNS system the mcb or rcbo is your method of complying with 411.3.2.2 & you use table 41.3
The rcd is for additional protection so 1667 does not come into it.
Table 41.5 is for TT systems.
If the maximum Zs value for a circuit in a TN system cannot be met, the circuit may be protected by a 30ma RCD. 531.3.1
I'm not sure that you can use that in a domestic situation Sintra, 531.3.1 does not deal with circuits but equipment in certain parts of a circuit.
So if you have a high Zs on a final ring circuit for example, you cannot just use the rcd as your main protection in my opinion.
Though I am of course open to being shown otherwise.
i must admit i prefer to get the readings withing the permitted levels
So if you have a high Zs on a final ring circuit for example, you cannot just use the rcd as your main protection in my opinion.
Absolutely.
Unless it's a TT system there is no reason at all not to have Zs well within limits. A good design leads to a compliant install without the need for an RCD to cover a bad one.
The amount of times I've heard "dont worry about that there's an RCD on it anyway"
I'm not sure that you can use that in a domestic situation Sintra, 531.3.1 does not deal with circuits but equipment in certain parts of a circuit.
So if you have a high Zs on a final ring circuit for example, you cannot just use the rcd as your main protection in my opinion.
Though I am of course open to being shown otherwise.
Agree with this also. On a new circuit it should never happen if designed correctly but it can be a quick fix on an existing circuit with a high Zs.
yes on tt system if ze less than 1667 then everythink ok.because rcd will disconnect in 0.2 thus satisfying 0.4 disconnection time
the maximum zs reading on a tn circuit incorporating an rcd shall be 1667 as per iee regs.therefore zs readings redundant on 99% of domestic installations
I would draw peoples attention to 411.4.9
reg 411.5.3 rcd protection 0.2 dissconection any system
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