High Zs with RCBO

[Dean Williams]

Hi folks
Just tested a convent/nursing home for nuns

found some long ring circuits frotected by 32A Type C RCBOs, the max Zs allowable is 0.72, but i was getting readings between 0.83 and 0.98.

Are these readings acceptable because of the use of RCBOs, i.e. does it matter what the reading is as an RCBO is likely to trip during a fault regardless

Or should a type B RCBO be fitted?

Note: its a TN-C-S system

thanks

 

[Deleted member 26818]

Unless there are devices with high start up currents on the ring, I'd replace with Type B to provide overcurrent protection.

 

[Dean Williams]

Thanks...just out of interest, someone told me that you can't get type B rcbo's made by MG/Schneider....is that true?

 

[Farmelectrics]

the fact that you have rcbos fitted means you can have a zs of 1667ohms so your readings are ok

 

[Jay Sparks]

the fact that you have rcbos fitted means you can have a zs of 1667ohms so your readings are ok

Was just about to say that. But they are right, I cant find the type b MG/Shcneider rcbo's anywere. I know Wylex do type b but dont think will fit MG/Shcneider board.

 

[SPARTYKUS]

No, disagree, fit the right one like spin says. Circuit should comply at dead test level even when its a TT so as long as can get one a type B then larfin'.

Rcd might fail (yes i know this is an rcbo) and then what. Crap resistance and an MCB thats not keen....

 

[Dean Williams]

thanks for your opinions, is there a definitive answer to this question though?

 

[Deleted member 26818]

To my mind yes.
RCDs are additional protection.
What you have here is a device which should provide both overcurrent and earth fault protection, it would seem to me a bit pointless using such a multipurpose device, if it does not do the job it is designed to do.

 

[SPARTYKUS]

Good post spinlondon, key word "additional"

 

[telectrix]

agree with spinlondon. but just a thought.... if possible, measure R1+R2. it has been known for RCBOs and RCDs to inflate the Zs readings due to their internal components.

 

[Chr!s]

Or it could just be a poor contact

 

[telectrix]

or poor design. was said that it was a long circuit, so maybe should have been wired in 4mm. that or should have used type B . it's not known why type C have been fitted. could have been just what was in stock needed using.

 

[SPARTYKUS]

or poor design. was said that it was a long circuit, so maybe should have been wired in 4mm. that or should have used type B . it's not known why type C have been fitted. could have been just what was in stock needed using.

Happens alot that does

 

[Deleted member 9648]

To my mind yes.
RCDs are additional protection....RCD's can also provide earth fault protection.411.5.2....411.4.9...,RCD's for additional protection only cover some equipment listed in 411.3.3
What you have here is a device which should provide both overcurrent and earth fault protection, it would seem to me a bit pointless using such a multipurpose device, if it does not do the job it is designed to do.It does provide both overcurrent and earth fault protection,overcurrent can be due to short circuit,not just earth fault[/QUOTE]


Rcd's also provide earth fault protection.....if they didnt a TT system would not be practical.....there is also a requirement for fault (short circuit)protection which a typeC overcurrent device will provide instantaneously,therefore it is not pointless. In this instance with the Zs 0f 0.98 a 32a type C will take approximately 8 seconds to disconnect an earth fault without the RCD,so even if the RCD fails a type C is not a complete waste of time.The Rcd will meet the disconnection times for earth fault protection.
I think if this is a result of a PIR it may be hard to justify the expense to the client of replacing RCBO's which already comply with the regulations,and provide satisfactory protection,based on a personal opinion,rather than a regulation.

 

[Deleted member 9648]

Good post spinlondon, key word "additional"

.....and incorrect in this context......refer to 411.3.3 for additional,and 411.4.9 for RCD's as earth fault protection.