High Zs with RCBO

[SPARTYKUS]

Wirepuller, 8 seconds instead of 0.1 or less... Not completely pointless butnot far off!

How much would another RCBO be that works properly? 20 spondoolicks

 

[nickblake]

MG / scneider are basically Square D same company bit like wylex volex and crabtree so yes they do do RCBO's in B type

 

[Deleted member 9648]

Wirepuller, 8 seconds instead of 0.1 or less... Not completely pointless butnot far off!

How much would another RCBO be that works properly? 20 spondoolicks

It does work properly....you are spending a clients money on your assumption that the device will fail....Do you meet the max Zs for an overcurrent device on a TT based on your view that an RCD is unreliable as a means of earth fault protection?,or do you condemn TT's? If you rely on an RCD for earth fault protection on a TT why is it unacceptable on a TN?

 

[Deleted member 9648]

Just to clarify here....on a newly installed TN circuit I would expect the Zs to meet the limits for the overcurrent device,as per Spin.
However,reading the OP it appears to be a PIR. The job of the inspector on a PIR is to highlight items which do not comply with 7671,not personal preferences.As the OP's circuits comply with 7671 I believe it is incorrect to recommend replacement of the RCBO's.

 

[tony mc]

Just out of intrest what was your R1+R2 readings if carried out as I had the same problem last week where the RCD was putting up the ZS reading when tested live just as Tel has mentioned.
Tested without going through RCD and readings as expected.

 

[Des 56]

It does work properly....you are spending a clients money on your assumption that the device will fail....Do you meet the max Zs for an overcurrent device on a TT based on your view that an RCD is unreliable as a means of earth fault protection?,or do you condemn TT's? If you rely on an RCD for earth fault protection on a TT why is it unacceptable on a TN?

I'm not taking issue with any of your comments, but am going to repeat something that I have stated before,I must have read something to this effect in the past that makes it pop up in my head ,its probably a load of Twaddle,but relevant if there is a smidgen of truth

On a TT system the magnitude of the earth fault current is much smaller than a Tn system
Rcds may have a higher percentage rate of failure because the magnitude of the earth fault current on the Tn system throws the Rcds electronics on a wobbly
A compliant Tn system would more than likely have the fault cleared by the overcurrent before any damaging large currents cause naughties to go on in the Rcds innards
The conclusion then being that a Tn system should have overcurrent devices on a compliant system rather than adopting TT system requirements

This is not my veiw,its a suggestion for debate or dismissal whatever,or explanation otherwise by the those more learned folk than I

 

[telectrix]

Just out of intrest what was your R1+R2 readings if carried out as I had the same problem last week where the RCD was putting up the ZS reading when tested live just as Tel has mentioned.
Tested without going through RCD and readings as expected.

i got my brains fried on a call-out recently where the service engineer for the gas boiler tolds the client that the Zs reading he had obtaind at the boiler FCU was unacceptably high. i measured it at 351ohms, this was a TNC-S installation. Ze read at 0.2. other circuits gave readings similar, around 350ohms. however a "Zs" measured at the L busbar in the CU read 349ohms. twas the dammned RCD ( front end on all circuits ) causing the problem.

 

[Deleted member 9648]

I'm not taking issue with any of your comments, but am going to repeat something that I have stated before,I must have read something to this effect in the past that makes it pop up in my head ,its probably a load of Twaddle,but relevant if there is a smidgen of truth

On a TT system the magnitude of the earth fault current is much smaller than a Tn system
Rcds may have a higher percentage rate of failure because the magnitude of the earth fault current on the Tn system throws the Rcds electronics on a wobbly
A compliant Tn system would more than likely have the fault cleared by the overcurrent before any damaging large currents cause naughties to go on in the Rcds innards
The conclusion then being that a Tn system should have overcurrent devices on a compliant system rather than adopting TT system requirements

This is not my veiw,its a suggestion for debate or dismissal whatever,or explanation otherwise by the those more learned folk than I

Not going to argue,but if that is the case it should be made a requirement for the Zs on a TN to be within that for the O/C device....at the moment that is not the case.

 

[SPARTYKUS]

Just to clarify here....on a newly installed TN circuit I would expect the Zs to meet the limits for the overcurrent device,as per Spin.
However,reading the OP it appears to be a PIR. The job of the inspector on a PIR is to highlight items which do not comply with 7671,not personal preferences.As the OP's circuits comply with 7671 I believe it is incorrect to recommend replacement of the RCBO's.

I read this and thought about it alot, and ...if thats the case then I agree. You cannot assume everything will fail and therefore must be replaced. At that point, as you say, if the RCBO is functioning as it's meant to, then the circuit is protected.

I just prefer to shy away from the "RCD will fix everything" idea and the inference that circuits no longer need to be designed and installed correctly because it no longer matters (and I am in no way suggesting that is what you are saying wirepuller!)

 

[Dean Williams]

gosh, all these different opinions. Where does the NICEIC stand on this?, i'll be contacting them on Monday anyway.

 

[Dean Williams]

I contacted the NICEIC this morning, he said the reading which was higher than the max value given by bs7671, was acceptable as the RBCO incorporates an RCD. and where it says max. ZS allowed on NIC form, if the measured reading is over 0.72, then to put 1667 in the box.

 

[SPARTYKUS]

SO

(and this is a general point aimed at forum members not you Dean)


does this then mean there is no need to design circuits correctly to BS7671 anymore with correct resistance values (R1+R2) as you can simply put an RCD in front and everything is ok and acceptable?

unk:

 

[telectrix]

SO

(and this is a general point aimed at forum members not you Dean)


does this then mean there is no need to design circuits correctly to BS7671 anymore with correct resistance values (R1+R2) as you can simply put an RCD in front and everything is ok and acceptable?

unk:

if it does go that way, then they can do away with all of us and get cheap immigrant labour to do all the work, from design to finish. no more need for testing, the RCD will do away with all that nonsense. certificates will be a half page multi choice. tick 1 box only:-

1. it works
2. it don't work
3. it went bang
4. who cares anyway

and the above can be the 4 codes on the new PIRs

 

[Deleted member 9648]

SO

(and this is a general point aimed at forum members not you Dean)


does this then mean there is no need to design circuits correctly to BS7671 anymore with correct resistance values (R1+R2) as you can simply put an RCD in front and everything is ok and acceptable?

unk:

No.......The Zs reading is only one test of whether a circuit is satisfactory. R1+R2,Ze,volt drop all have to be within the parameters of good circuit design.....Dont lose sight of the fact that in the OP's example the only reason the Zs exceeds the maximum for the overcurrent device is because it is a type C......Change it for a type B if you want, but the circuit will be exactly the same.....Given that the RCD provides earth fault protection changing the overcurrent device seems to me pointless.