IET logo
 
IET
Decrease font size
Increase font size
Topic Title: RCD's In Parallel
Topic Summary: Protecting sub-mains
Created On: 10 October 2012 07:58 AM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 10 October 2012 07:58 AM
User is offline View Users Profile Print this message



primo

Posts: 426
Joined: 10 January 2008

Has anyone come across several RCD's in parallel to protect sub-mains and if so are there any issues to be aware of with this set up.

I'm thinking about possibly using 3x 100mA time delay RCD's fed from one supply to feed 3 sub-mains on a TT installation. Essentially creating a split load I suppose. The idea would be that a N-E fault on one of the sub-mains final circuits would not take out a sole up front 100mA TD RCD thus leaving the other sub-mains / circuits energised.

I was called to a fault the other day where a 300mA up front device was tripping due to a N-E fault on a circuit fed by a 30mA RCBO. The other RCBO's in this board were also tripping / no re-setting in various combinations until the N-E fault was identified. This got me thinking about the current set up I am doing.

So, any probs with 3 or 4 100mA TD RCD's fed from a Helnley supplying sub-mains to individual boards which will all have 30mA RCBO's in?

I suppose my concern is that after going to all that trouble, a fault on one circuit could still trouble the up front RCD of another sub-main if the fault takes a non-linear path?!

Thanks.
 10 October 2012 08:25 AM
User is offline View Users Profile Print this message


Avatar for sparkiemike.
sparkiemike

Posts: 1513
Joined: 24 January 2008

Is the issue that the RCBOs are single pole, so even if they operate the Neutral still connected providing a path back to the upstream RCD and causing an imbalance hence the upstream RCD operating
 10 October 2012 08:56 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 18919
Joined: 23 March 2004

So, any probs with 3 or 4 100mA TD RCD's fed from a Helnley supplying sub-mains to individual boards which will all have 30mA RCBO's in?


yes, unless you have double or all pole RCBO's they won't disconnect N-E faults and the upstream 100mA S type will still operate - ie you will not have achieved the design intent.

There is nothing wrong (in principle) with your design (think of it as mains into a busbar (your henly blocks) and then your submains each having a source end RCD (and hopefully fuse or circuit breaker) protection. If you substitute the RCBO's for a board RCD (which is double pole then all will be OK, if you use double pole RCBO's, ditto - it's the single pole nature of RCBO's that creates the selectivity problem - coupled with the often disabling effects of N-E faults

Regards

OMS

-------------------------
Failure is always an option
 10 October 2012 05:35 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3374
Joined: 22 November 2007

Also, don't forget (which I'm sure you wont) overcurrent protection on the sub mains.

If it was my design, if the sub boards needed to be RCBO and couldn't be dual or single rcd boards, I'd be looking at a TD incoming device in each board and an adjustable one to protect the submains.


Stu
 10 October 2012 06:06 PM
User is offline View Users Profile Print this message



peteTLM

Posts: 3083
Joined: 31 March 2005

Originally posted by: primo

Has anyone come across several RCD's in parallel to protect sub-mains and if so are there any issues to be aware of with this set up.



I'm thinking about possibly using 3x 100mA time delay RCD's fed from one supply to feed 3 sub-mains on a TT installation. Essentially creating a split load I suppose. The idea would be that a N-E fault on one of the sub-mains final circuits would not take out a sole up front 100mA TD RCD thus leaving the other sub-mains / circuits energised.



I was called to a fault the other day where a 300mA up front device was tripping due to a N-E fault on a circuit fed by a 30mA RCBO. The other RCBO's in this board were also tripping / no re-setting in various combinations until the N-E fault was identified. This got me thinking about the current set up I am doing.



So, any probs with 3 or 4 100mA TD RCD's fed from a Helnley supplying sub-mains to individual boards which will all have 30mA RCBO's in?



I suppose my concern is that after going to all that trouble, a fault on one circuit could still trouble the up front RCD of another sub-main if the fault takes a non-linear path?!



Thanks.


If i was going to construct this, id use either normal TD RCD's with a suitable MCB next to it, or opt for a manufactured item such as a vigi unit, all inside a din rail enclosure. Got to be neater than a henley- multiple garage DB nightmare.

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 10 October 2012 08:22 PM
User is offline View Users Profile Print this message



primo

Posts: 426
Joined: 10 January 2008

Originally posted by: OMS

So, any probs with 3 or 4 100mA TD RCD's fed from a Helnley supplying sub-mains to individual boards which will all have 30mA RCBO's in?




yes, unless you have double or all pole RCBO's they won't disconnect N-E faults and the upstream 100mA S type will still operate - ie you will not have achieved the design intent.



There is nothing wrong (in principle) with your design (think of it as mains into a busbar (your henly blocks) and then your submains each having a source end RCD (and hopefully fuse or circuit breaker) protection. If you substitute the RCBO's for a board RCD (which is double pole then all will be OK, if you use double pole RCBO's, ditto - it's the single pole nature of RCBO's that creates the selectivity problem - coupled with the often disabling effects of N-E faults



Regards



OMS


My design intent is that a N-E fault on one of the sub-mains final circuits will not cause the up front TD RCD of one of the other sub-mains to operate.

Ideally I would have DP RCBO's or 30mA RCD incomer for the sub-main boards. The install is still evolving so may end up with an arrangement of this sort although at present the new work is 100mA TD with 30mA RCBO final circuits, the rest is existing and will get sorted so final dis board arrangements are still being designed!

It is, as you say, the single pole nature of the RCBO's that has got me thinking about the design.
 10 October 2012 08:25 PM
User is offline View Users Profile Print this message



primo

Posts: 426
Joined: 10 January 2008

Originally posted by: slittle

Also, don't forget (which I'm sure you wont) overcurrent protection on the sub mains.



If it was my design, if the sub boards needed to be RCBO and couldn't be dual or single rcd boards, I'd be looking at a TD incoming device in each board and an adjustable one to protect the submains.





Stu


I haven't forgotten about over current protection, honest!

That's a nice idea to have TD incomer but what adjustable device are you talking about for the sub-main protection. This is a large domestic (with 2 phases coming in, but that's irrelevant for this discussion!) so I don't really want to be getting into shunt trips and earth leakage devices for MCCB's etc!
 10 October 2012 08:28 PM
User is offline View Users Profile Print this message



primo

Posts: 426
Joined: 10 January 2008

Originally posted by: peteTLM

Originally posted by: primo



Has anyone come across several RCD's in parallel to protect sub-mains and if so are there any issues to be aware of with this set up.







I'm thinking about possibly using 3x 100mA time delay RCD's fed from one supply to feed 3 sub-mains on a TT installation. Essentially creating a split load I suppose. The idea would be that a N-E fault on one of the sub-mains final circuits would not take out a sole up front 100mA TD RCD thus leaving the other sub-mains / circuits energised.







I was called to a fault the other day where a 300mA up front device was tripping due to a N-E fault on a circuit fed by a 30mA RCBO. The other RCBO's in this board were also tripping / no re-setting in various combinations until the N-E fault was identified. This got me thinking about the current set up I am doing.







So, any probs with 3 or 4 100mA TD RCD's fed from a Helnley supplying sub-mains to individual boards which will all have 30mA RCBO's in?







I suppose my concern is that after going to all that trouble, a fault on one circuit could still trouble the up front RCD of another sub-main if the fault takes a non-linear path?!







Thanks.




If i was going to construct this, id use either normal TD RCD's with a suitable MCB next to it, or opt for a manufactured item such as a vigi unit, all inside a din rail enclosure. Got to be neater than a henley- multiple garage DB nightmare.


You're right, it already looks like a bit of 25mm2 spaghetti junction, and I was honestly thinking about creating an enclosure with all of the TD RCD's and MCB's in it, apart from the switch fuse for the big one!

I'll have a look at Vigi, thanks.
 10 October 2012 09:05 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3374
Joined: 22 November 2007

If you look at the hager version of the vigi module (can't remember the part number but could find it) they make an adjustable one (as possibly do others). You can set both trip current and delay time on the hager one and then protect the settings with a "sealable" flap.

I've used them to protect swa submains that then become proper hard drawn overheads and then feed board with TD incomers, so far (3 years on) no problems.

If you are using combined modules or separates why do you want a switch fuse ??, remember as well if the installation is TT from the start you won't be loved if you have a metal switch fuse before the RCD's.

Stu
 10 October 2012 10:18 PM
User is offline View Users Profile Print this message



primo

Posts: 426
Joined: 10 January 2008

Originally posted by: slittle

If you look at the hager version of the vigi module (can't remember the part number but could find it) they make an adjustable one (as possibly do others). You can set both trip current and delay time on the hager one and then protect the settings with a "sealable" flap.



I've used them to protect swa submains that then become proper hard drawn overheads and then feed board with TD incomers, so far (3 years on) no problems.



If you are using combined modules or separates why do you want a switch fuse ??, remember as well if the installation is TT from the start you won't be loved if you have a metal switch fuse before the RCD's.



Stu


I'm at the early stages of adding onto / re-wiring an existing TT installation.

I have installed a 100mA TD up front, before the metal switch fuse!
This is the same install as the 10mA RCBO stables one that you have helped me on!

It's a switch fuse due to the lack of planning time to get this part up and running and the 35mm2 SWA (long run to stables and future PV at that location) that I don't fancy terminating into a domestic enclosure at the supply end (no 6" galv there...yet).

So, if you have TD incomers at the boards, what settings do you you use for the vigi mods, assuming a TT origin, to protect the sub-main, and why would you need the TD incomer if it is protected up stream at the start of the sub-main? Is it just for convenience to re-set / test if there is a fault and also to protect the sub-main?
 10 October 2012 10:39 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3374
Joined: 22 November 2007

Very quickly because it's past my bed time...

I sort of thought it might be the same job, if you've got the 100ma TD incomer then the vigi modules or TD rcd's are not going to help you as they won't grade with your incoming device.

If it was mine, I'd go with what you've got, protect the submains as necessary for overload and RCD the outlying boards accordingly.

Unless you change the incoming device for something with say 300ma and 1 second trip time, you are not going to gain anything from the TD on each submain.

Stu
 10 October 2012 11:36 PM
User is offline View Users Profile Print this message



primo

Posts: 426
Joined: 10 January 2008

Originally posted by: sparkiemike

Is the issue that the RCBOs are single pole, so even if they operate the Neutral still connected providing a path back to the upstream RCD and causing an imbalance hence the upstream RCD operating



No and yes!
Just wanted to know of there was any way a fault may operate another RCD of the same type that was in parallel. Not too fused if the upstream device operates with a NE fault, main aim is to ensure none of the other sub-mains are affected by a fault one. The upstream devices are there purely to give the 1s disconnection time for the sub- mains.
 10 October 2012 11:40 PM
User is offline View Users Profile Print this message



primo

Posts: 426
Joined: 10 January 2008

Originally posted by: slittle

Very quickly because it's past my bed time...



I sort of thought it might be the same job, if you've got the 100ma TD incomer then the vigi modules or TD rcd's are not going to help you as they won't grade with your incoming device.



If it was mine, I'd go with what you've got, protect the submains as necessary for overload and RCD the outlying boards accordingly.



Unless you change the incoming device for something with say 300ma and 1 second trip time, you are not going to gain anything from the TD on each submain.



Stu


Thanks. But what I'm proposing is 100mA TD for each sub just to give the 1s discon time rather than just one for all sun mains whilst keeping each sub on in the event of a NE fault on any one.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.