Type D RCBOs

[johnnycash126]

Dear all,

I have to protect a 13 amp ring main which is currently protected by a type b rcbo.The circuit is going to be used by step down transformers from the US.Notorious for tripping on start up.I thought a change to a type D rcbo would cure this,however neither MEM or Crabtree make a type D RCBO.This makes me think that perhaps im not allowed to do this.

Anyone any experience of this?

 

[Deleted member 9648]

Hmmmm!.....while all that may be the case mate 411.4.4 is good enough for my purposes. For those without access to the BGB 411.4.4 lists devices which may be used for fault protection on a TN system as (i) An overcurrent protective device....and (ii) An RCD

 

[R G]

In a TT system due to the high values you can expect when you have a fault current to earth it is thus limited in ampage and will have low effect to the voltage drop during the fault.. this will be sufficient to operate the device within specified times.

In TNS and TNCS due to the much lower resistance values by simple ohms law you can expect earth fault current in many orders of 1000amps and at these current levels for the duration of the fault it can be the case that voltage drop below 50v and can impair/delay the operation of a earth leakage device in effect so it wouldn't comply ... this is why you find rcbo's listed in table 41.3 because the Overload nature of the device will still ensure it functions within time even with the voltage drop.
This brings me back to fitting rcd's to back up a design flaw in a tns or tncs set-up they may not still meet requirements during an actual fault although it is noted their implementation is more beneficial than not to hence it seems the depth of this is not often explained.

So as i said changing the Rcbo to either a mcb with separate rcd or a type D rcbo should still comply with table 41.3 ...fitting RCD protection to comply with disconnection times where Zs is not met must be a last resort either because it exists already and your affording extra protection off the back of a EICR or you have a TT set-up in which case the RCD won't see the voltage issues of a tns or tncs.

Hope you see why i said the OP shouldn't be introducing a failed zs where it complied before and hence with the limited info we have a separate circuit is the only advice that is safe to offer until we get more info.

Thanks for that and definiatelly needed more info...

 

[Darkwood]

I can get away from domestic too long and kind of get industrialised so i can be quite ---- (not in a gay way :biker about design and upgrades ... it may be the case the regs are a little more relaxed domestically surrounded this issue but until im shown good evidence to back up i will assume the above as i have explained is correct ... i am used to installations many orders larger than domestic as you can guess but at the root of it all the maths and theory are the same.... Just ive always been taught that in a TNCS there should be little reason Zs isn't met and most of the time in TNS systems, usually supply earth issues are to blame here though.

 

[Darkwood]

Hmmmm!.....while all that may be the case mate 411.4.4 is good enough for my purposes. For those without access to the BGB 411.4.4 lists devices which may be used for fault protection on a TN system as (i) An overcurrent protective device....and (ii) An RCD

Your taking this regulation out of context its only stating the options available whether they comply is down to the other regulation within the BGB ..... its akin to saying you can stop you wheels turning on your car with either the foot brake or the hand brake... using each option at the correct time is crucial to safety..you wouldn't pull your handbrake at 70mph :O/ .. this is the biggest issue with the BGB a lot of regs are down to interpretation i put it in the same context as the Quran - in the wrong hands its a very dangerous book in deed...

 

[Engineer54]

One of the biggest scourges in the electrical industry today!! Since RCD devices have become common place. (...Eg use of the things as an all purpose cover all device) why even bother to attain or maintain suitable installation/circuit Zs values when you can just bung an RCD in!! No-one gives even a glancing thought, as to what happens if and when the RCD fails to operate!! ..After all they are such reliable bits of of kit aren't they, ...NOT!!

 

[Deleted member 9648]

One of the biggest scourges in the electrical industry today!! Since RCD devices have become common place. (...Eg use of the things as an all purpose cover all device) why even bother to attain or maintain suitable installation/circuit Zs values when you can just bung an RCD in!! No-one gives even a glancing thought, as to what happens if and when the RCD fails to operate!! ..After all they are such reliable bits of of kit aren't they, ...NOT!!

I understand what you are saying here I really do,but I think it is not entirely relevant to the OP's scenario.
It appears an existing circuit is being used to supply equipment with a high inrush current which was not envisaged when that circuit was originally installed. All that is required is to change the characteristics of the OCPD in order to prevent nuisance tripping. As the required disconnection time is no longer met by the OCPD an RCD can be used to achieve this. Lets assume said RCD fails. Assume the required Zs of 0.36 for a type D mcb is exceeded to a value of say 0.6 ohms,which would be a typical value for an average TN fed ring final.That would give a fault current of 383a...and a disconnection time of around 4s. So the OCPD will still operate in the unlikely event of the RCD not operating.
It seems ridiculous to me to consider new circuits/rewiring when a simple solution is available.Nobody is suggesting the use of RCD's to bypass good design and maximum Zs on new or altered circuits....just a way of making an existing circuit suitable for a change in use.

 

[Engineer54]

I understand what you are saying here I really do,but I think it is not entirely relevant to the OP's scenario.
It appears an existing circuit is being used to supply equipment with a high inrush current which was not envisaged when that circuit was originally installed. All that is required is to change the characteristics of the OCPD in order to prevent nuisance tripping. As the required disconnection time is no longer met by the OCPD an RCD can be used to achieve this. Lets assume said RCD fails. Assume the required Zs of 0.36 for a type D mcb is exceeded to a value of say 0.6 ohms,which would be a typical value for an average TN fed ring final.That would give a fault current of 383a...and a disconnection time of around 4s. So the OCPD will still operate in the unlikely event of the RCD not operating.
It seems ridiculous to me to consider new circuits/rewiring when a simple solution is available.Nobody is suggesting the use of RCD's to bypass good design and maximum Zs on new or altered circuits....just a way of making an existing circuit suitable for a change in use.

Sorry i wasn't specifically referring to the OP's scenario, basically just making a blanket statement on what seems to be going on out there with regards as to using RCD devices as the cure all solution to every Zs problem....

 

[GLENNSPARK]

One of the biggest scourges in the electrical industry today!! Since RCD devices have become common place. (...Eg use of the things as an all purpose cover all device) why even bother to attain or maintain suitable installation/circuit Zs values when you can just bung an RCD in!! No-one gives even a glancing thought, as to what happens if and when the RCD fails to operate!! ..After all they are such reliable bits of of kit aren't they, ...NOT!!

its poor design...short n sweet.

this is where i take issue with BS7671 for advocating it as well.....

 

[Deleted member 9648]

its poor design...short n sweet.

this is where i take issue with BS7671 for advocating it as well.....

So why in a recent thread were you accepting Zs values in the region of 20 ohms? Why is it acceptable to move the goalposts and rely on means other than an OCPD on a TT system?

 

[GLENNSPARK]

So why in a recent thread were you accepting Zs values in the region of 20 ohms? Why is it acceptable to move the goalposts and rely on means other than an OCPD on a TT system?

yes...i agree with you on this...

however i have been back to that property several times (like i also said i would)...and have watched it come down

last measurement was in the region of 11 ohms...

the point as well is that i said i was looking for a low stable value...didn`t I...

 

[Rockingit]

Back to the OP's issue, however, of preventing in-rush tripping issues..... OP, I have a sneaky suspicion at the back of my mind that this sounds like an events industry related query??? And it's a temp install????

 

[Engineer54]

So why in a recent thread were you accepting Zs values in the region of 20 ohms? Why is it acceptable to move the goalposts and rely on means other than an OCPD on a TT system?

You're now twisting things out of context!! This just shows how contradictory BS 7671 can be, depending on how your looking at things and what you want to do. I say give the whole thing over to CIBSE, never seen a bad publication from them and they are far more in tune with the building and construction industry than ITE!!

Makes you think, when the Yanks insist on another rod being sunk if the Ra value is over 20/25 ohms, and they aren't talking about 1.2m 3/8'' rods, they are talking 10 foot rods 5/8''or 3/4'' rods!! wheres BS 7671 is talking about 200 ohms. ....Someone has it Very WRONG, and in this case it sure aint the Yanks!! ...lol!!

 

[Silly Sausage]

........................this is the biggest issue with the BGB a lot of regs are down to interpretation i put it in the same context as the Quran - in the wrong hands its a very dangerous book in deed...

Hahahaha...great analogy! :smilielol5:

 

[Deleted member 9648]

yes...i agree with you on this...

however i have been back to that property several times (like i also said i would)...and have watched it come down

last measurement was in the region of 11 ohms...

the point as well is that i said i was looking for a low stable value...didn`t I...

You're now twisting things out of context!! This just shows how contradictory BS 7671 can be, depending on how your looking at things and what you want to do. I say give the whole thing over to CIBSE, never seen a bad publication from them and they are far more in tune with the building and construction industry than ITE!!

Makes you think, when the Yanks insist on another rod being sunk if the Ra value is over 20/25 ohms, and they aren't talking about 1.2m 3/8'' rods, they are talking 10 foot rods 5/8''or 3/4'' rods!! wheres BS 7671 is talking about 200 ohms. ....Someone has it Very WRONG, and in this case it sure aint the Yanks!! ...lol!!

The point I was making was nothing to do with Ra values,which is another matter entirely. The point was simply that the gist of this thread is using an RCD to meet disconnection times on a TN system is always ,regardless of circumstances, unacceptable.....and yet most of the posters would have no problem with using an RCD to meet disconnection times on a TT.
Doesn't make sense.

 

[Darkwood]

The point I was making was nothing to do with Ra values,which is another matter entirely. The point was simply that the gist of this thread is using an RCD to meet disconnection times on a TN system is always ,regardless of circumstances, unacceptable.....and yet most of the posters would have no problem with using an RCD to meet disconnection times on a TT.
Doesn't make sense.

Ill remind you my post no' 30 explains this and why RCD's on a TT are reliable due to limited volts drop but on a TN system the much higher current can render the RCD useless due to VD in fault condition hence the O/L nature of the device must then trip within the time limit - this can only be achieved if the Zs is met.
A TT system requires a different plane of thought and should not be a comparison.