erm, try table 41.5 !
Max Zs entry on EIC for an RCD circuit
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erm, try table 41.5 !
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How strange, table 41.5 gives a touch voltage of over 50v (50.01v) 
So a final circuit is being protected by both an MCB and an RCD and the circuit has a measured Zs low enough to trip the MCB within 0.4s and the RCD test showed 5*I of 20ms (0.02s) then the RCD will go first, only if it is an earth fault. If there is a short circuit fault then the MCB will go as the RCD will detect nothing wrong. Am I correct.
Also if there is an overload due to to many heaters being plugged in then the RCD will do nothing and the MCB will trip according to the graph on page 249 in BS 7671:2008
Also if there is an overload due to to many heaters being plugged in then the RCD will do nothing and the MCB will trip according to the graph on page 249 in BS 7671:2008
Garymo: I think 1667 took less space than 1666.66666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666....etc
podooser: Yes, sounds correct to me. other than the measured Zs needs only be low enough to satisfy the 50V rule (1666 and a bit ), not necessarily low enough to disconnect in 0.4 seconds as that is being achieved by the RCD.
podooser: Yes, sounds correct to me. other than the measured Zs needs only be low enough to satisfy the 50V rule (1666 and a bit
), not necessarily low enough to disconnect in 0.4 seconds as that is being achieved by the RCD.Sponsored Links
Garymo: I think 1667 took less space than 1666.66666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666....etc
Maybe, but it's incorrect (over 50v).
Now 1666 sounds better and is under 50v
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Forgive me guys, I don't have the book with me. Please help me out.
Does it actually say that 1666 should be the figure entered on certs???
Does it actually say that 1666 should be the figure entered on certs???
In my opinion when completing an EIC, and I have to fill in a value that asks me to enter data from current BS7671 wiring regs, I generally tend to enter the data as published. i.e. if table 41.5 dictates that the maximum value os Zs in this instance is 1667, then I feel inclined to enter the value of 1667. Why do you seem hell bent on disagreeing with me on this point when its been quite clearly pointed out to you. You seem an intelligent and highly qualified individual, I just wonder why you can't grasp this particular point. Tell you what, if I'm wrong I'll never post on here again !
Garymo: I think 1667 took less space than 1666.66666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666....etc
Maybe, but it's incorrect (over 50v).
Now 1666 sounds better and is under 50v
Sounds better ? why ? Its written in black and white in the regs so how can it be wrong ? Because its .01 V above V=IR <50V ? It may be contradictory, but that is the MAXIMUM value. in the book. Funny but a few days ago you were entering incorrect data and then also reducing that by 20% because thats what the man from the ECA told you to do !!
Garymo: I think 1667 took less space than 1666.66666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666....etc
Maybe, but it's incorrect (over 50v).
Now 1666 sounds better and is under 50v
Sounds better ? why ? Its written in black and white in the regs so how can it be wrong ? Because its .01 V above V=IR <50V ? It may be contradictory, but that is the MAXIMUM value. in the book. Funny but a few days ago you were entering incorrect data and then also reducing that by 20% because thats what the man from the ECA told you to do !!
Sounds better than 1666.666666666666666666666666666666666666666666, not 1667.
I believe you should round down and not up in circumstances like these - to err on the side of caution.
Just because it's in black and white doesn't make it right. 1667 allows touch voltage to rise above 50v - FACT.
Are you having a little dig at me?
Yes I entered 80% values because me scheme provider told me that was the correct thing to do. I now enter 100% values because I'm now told different.
Does it really matter? No, it doesn't. I entered values lower than that required (80%). There would have been more of a problem if I entered figures higher than required. Anyway, I still think the 80% values are more of a real world value considering the cable isn't going to stay at ambient temperatures during a fault. Temperature will rise rapidly and with that impedance of the cable will fall so the 100% value has just gone out of the window.
Anyway, you say I was entering incorrect data and then reducing that by 20% - I say get your facts right.
I took the CORRECT data and reduced that by 20% and entered that figure.
What does it matter?, I'd hope that anyone that calls themselves an electrician who sees any figure anywhere near 1666.666r on any type of TN system will be looking for a problem
Agreed.
I'll also carry on entering figures for the overcurrent characteristics of the RCBO from table 41.3.
Rightly or wrongly, there's no chance I'm going to compare measured or calculated earth fault loop impedance figures against 1667ohms.
Think about it logically. To measure anywhere near 1667 ohms on a TN system there's got to be something wrong. Either Ze is 1665 ohms - PROBLEM!! Or there's a measured R1+R2 of 1666.2 or higher - PROBLEM!!
It seems very silly to me.
Garymo: I think 1667 took less space than 1666.66666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666666....etc
Maybe, but it's incorrect (over 50v).
Now 1666 sounds better and is under 50v
Sounds better ? why ? Its written in black and white in the regs so how can it be wrong ? Because its .01 V above V=IR <50V ? It may be contradictory, but that is the MAXIMUM value. in the book. Funny but a few days ago you were entering incorrect data and then also reducing that by 20% because thats what the man from the ECA told you to do !!
Sounds better than 1666.666666666666666666666666666666666666666666, not 1667.
I believe you should round down and not up in circumstances like these - to err on the side of caution.
Just because it's in black and white doesn't make it right. 1667 allows touch voltage to rise above 50v - FACT.
Are you having a little dig at me?
Yes I entered 80% values because me scheme provider told me that was the correct thing to do. I now enter 100% values because I'm now told different.
Does it really matter? No, it doesn't. I entered values lower than that required (80%). There would have been more of a problem if I entered figures higher than required. Anyway, I still think the 80% values are more of a real world value considering the cable isn't going to stay at ambient temperatures during a fault. Temperature will rise rapidly and with that impedance of the cable will fall so the 100% value has just gone out of the window.
Anyway, you say I was entering incorrect data and then reducing that by 20% - I say get your facts right.
I took the CORRECT data and reduced that by 20% and entered that figure.
No not having a dig at you. My facts are right. I said you were entering incorrect data on the EIC for maximum Zs permitted. This is because you were not entering 1667, you were entering some other value, which is not the maximum. You were then applying a correction factor to this value for ambient temperature. Of course this is not unsafe, just that the circtificate is incorrect. Its only a small point and maybe it doesn't matter, the thread is four pages long because nobody seems to agree with me but is yet to prove me wrong !
What does it matter?, I'd hope that anyone that calls themselves an electrician who sees any figure anywhere near 1666.666r on any type of TN system will be looking for a problem
Agreed.
I'll also carry on entering figures for the overcurrent characteristics of the RCBO from table 41.3.
Rightly or wrongly, there's no chance I'm going to compare measured or calculated earth fault loop impedance figures against 1667ohms.
Think about it logically. To measure anywhere near 1667 ohms on a TN system there's got to be something wrong. Either Ze is 1665 ohms - PROBLEM!! Or there's a measured R1+R2 of 1666.2 or higher - PROBLEM!!
It seems very silly to me.
Garymo/Spark123 you seem to have completey missed the point. My OP
relates to the entry on the EIC for a maximum value, not what you measure, what is wrong, what you expect, what you would be happy with, what is acceptable or whatever. What's it matter ? I think it matters that an EIC is completed correctly !
I'm arguing with the whole scenario.
BS7671:2008 states a maximum of 1667 ohms (laughable because it exceeds the maximum touch voltage of 50v) yet realistically you could never have real world figures anywhere near that dues to an excessive Ze, poor connections in the line or cpc conductors or volt-drop out of spec.
BS7671:2008 states a maximum of 1667 ohms (laughable because it exceeds the maximum touch voltage of 50v) yet realistically you could never have real world figures anywhere near that dues to an excessive Ze, poor connections in the line or cpc conductors or volt-drop out of spec.
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I tend to agree! You have told us that the regs state a figure of 1667 should be entered as the maximum Zs allowed by BS7671:2008 on an EIC.
Now, my personal opinion of this is that IF a Zs cannot be achieved within normal limits (ie with circuits connected to TT systems), then I feel the 1666 maximum is correct. I don't agree with 1667. It may just be .01 of a volt over, but over it is.
I think this is another "missed" mistake, not corrected by the recently released corrigendum.
However (and I know an OSG has not yet been published for 7671:2008) the old OSG advised a maximum figure of 200 for TT.
Given this ambiguity, and where "normal" Zs limits can be adhered to (ie with TN systems), I believe that figures should be entered on the cert should be the lower figures.
Call me wrong if you like, but I don't feel entering "maximum allowed" figures which are lower is a problem.
Higher, yes, of course, but lower, no.
Now, my personal opinion of this is that IF a Zs cannot be achieved within normal limits (ie with circuits connected to TT systems), then I feel the 1666 maximum is correct. I don't agree with 1667. It may just be .01 of a volt over, but over it is.
I think this is another "missed" mistake, not corrected by the recently released corrigendum.
However (and I know an OSG has not yet been published for 7671:2008) the old OSG advised a maximum figure of 200 for TT.
Given this ambiguity, and where "normal" Zs limits can be adhered to (ie with TN systems), I believe that figures should be entered on the cert should be the lower figures.
Call me wrong if you like, but I don't feel entering "maximum allowed" figures which are lower is a problem.
Higher, yes, of course, but lower, no.
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