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?

 

[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.

 

[spark 68]

Care to explain more about the VD aspect DW ?

As I understood it modern RCD/RCBO being current operated, even if the VD falls momentarily to below 50V you would still have a massive current flow through one of the (L) main coils (on a SP RCD) to earth causing the trip coil to operate.

I am just having a think on 3 and 4 pole versions now.

I am not saying you are wrong btw, just interested in this aspect.

Edit: Having thought about the 3 and 4 pole versions, although you may indeed get a high VD, this correspondingly high current flow would not be returning via either another phase/s or the N through the magnetic core thus causing again a massive magnetic imbalance. hmm.

Ps. I don't really want to get into the high Zs/ poor design debate.

 

[Engineer54]

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.

There isn't much choice in the matter having to use an RCD to meet disconnection times in TT systems, especially UK TT systems!! .... Doesn't make the RCD device's any more reliable though!!

 

[GLENNSPARK]

There isn't much choice in the matter having to use an RCD to meet disconnection times in TT systems, especially UK TT systems!! .... Doesn't make the RCD device's any more reliable though!!

but you can still get as low as possible....where you know that there isn`t going to be wild fluctuations due to soil condition changes by getting under that layer...

and this means depth..

depth is key...with TT

 

[Engineer54]

but you can still get as low as possible....where you know that there isn`t going to be wild fluctuations due to soil condition changes by getting under that layer...

and this means depth..

depth is key...with TT

Ah, but alas you'll never convince wirepuller of anything to do with a TT systems stability i'm afraid!!

 

[Darkwood]

At Sparks 68

There are many aspects of rcbo operation and theory that go very deep ...

I can explain most of them from working voltages to earth leakage masking caused by harmonics but im so tired ill only get head mush ... most operating safety devices come with a working voltage range if your voltage is out of this range then the device won't be guaranteed to function correctly sometimes its for other reasons non related to fault conditions like alot of rcd's have a minimum voltage specified due to the testing function relying on a resistor but bare with me ill get back to you on this when i can keep my eyes open.

 

[spark 68]

Okay, later's DW.

 

[Deleted member 9648]

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.

That is your interpretation. But I'm an electrician,I work to Bs 7671.That is what I was taught in my apprenticeship,and if it ever goes ---- up that is what will be waved in my face in court. And Bs 7671 permits the use of an RCD for fault protection on a TN system.

 

[Deleted member 9648]

Ah, but alas you'll never convince wirepuller of anything to do with a TT systems stability i'm afraid!!

If you mean 20 ohms is good and 200 ohms isnt?.....then no you wont ever convince me!!

 

[Badged01]

Interesting debate, even made and old thicky like me think about things -- and get an appreciation of a few points I didnt understand so well as I thought! Guess it all boils down to if things go wrong and someone is gunning for your blood how you justify yourself in a court of law for your work.

BS7671 is guidance and meeting its requirements "are regarded by the HSE as likely to achieve conformity with the relevant parts of the EAWR's" but dont guarantee immunity. But not meeting them leaves you more exposed then meeting them, unless you have a very strong technical justification/experience (all provable) to roll-out in your defence and counter the prosecutions 'technical' experts! (Ive seen the defence hammered once by a technical expert and it wasnt a pretty sight)

You pays your money and you takes your choice, but if I was infront of the beak in a British court of law I think Id prefer to be relying on hanging my hat on having met BS7671, in whichever way you want to interpret it .......

 

[Darkwood]

This is an update from a company that provides an electronic PIR (as it was known) form that was updated to get rid of a glitch where using RCBO's defaulted the requirements to 1667ohms.. the amemdment now flags rcbo's on a TN system that do not meet requirements in accordance with 41.3.

http://blog.pirform.co.uk/post/2009/12/18/RCD-additional-intelligence-added.aspx

Like we have been saying throughout this thread the 1667 limit is applicable to TT only and if your on a tn system then your rcbo has to meet requirements of table 41.3.

Ill dig deeper into the exact reasoning Ive already attempted to explain in my words but either way it reflects the message to the OP and some posters in that Zs should be met in a TN system and rcd protection is not a cheap fix it option to give you a get out of jail card.

We are taught many things at college and this subject only cam about in the 16th then to the 17th ... i trained on the 15th and have always researched all new regulations to their core... most just read and accept amendments without understanding the core reasons for them... this on my mind opens the flood gates to misinterpretation and good debates like this one....

I don't claim to be any kind of know it all and do sometimes surprise myself to what i thought i knew was wrong but it is down to interpretation and until you are challenged you can plod along happily unaware hence i think this level of debate is on forums like this is a good thing.. not to prove who knows it all but to get to know the core values on what is your work bible.

I have self taught all amendments post 15th edition but rather than a poor tutor I have several forums to check my thinking!

 

[Engineer54]

If you mean 20 ohms is good and 200 ohms isnt?.....then no you wont ever convince me!!

No i specifically stated ''Stability''!!


As for your point above, ....Give me the 20 ohm value over you're numbty BS 7671 200 ohms every single day of the week!!

 

[Deleted member 9648]

This is an update from a company that provides an electronic PIR (as it was known) form that was updated to get rid of a glitch where using RCBO's defaulted the requirements to 1667ohms.. the amemdment now flags rcbo's on a TN system that do not meet requirements in accordance with 41.3.

PIRform | RCD additional intelligence added

Like we have been saying throughout this thread the 1667 limit is applicable to TT only and if your on a tn system then your rcbo has to meet requirements of table 41.3.

Ill dig deeper into the exact reasoning Ive already attempted to explain in my words but either way it reflects the message to the OP and some posters in that Zs should be met in a TN system and rcd protection is not a cheap fix it option to give you a get out of jail card.

We are taught many things at college and this subject only cam about in the 16th then to the 17th ... i trained on the 15th and have always researched all new regulations to their core... most just read and accept amendments without understanding the core reasons for them... this on my mind opens the flood gates to misinterpretation and good debates like this one....

I don't claim to be any kind of know it all and do sometimes surprise myself to what i thought i knew was wrong but it is down to interpretation and until you are challenged you can plod along happily unaware hence i think this level of debate is on forums like this is a good thing.. not to prove who knows it all but to get to know the core values on what is your work bible.

I have self taught all amendments post 15th edition but rather than a poor tutor I have several forums to check my thinking!

Home now and I dont have the BGB with me,but unless it has changed since the BRB the above statement is not true.
The reg in the BRB that applies if I recall is 411.4.9

If anyone has access to the BGB and can check if 411.4.9 is still there it would clear this up. If 411.4.9 has been removed since the BRB I will stand corrected.

 

[GLENNSPARK]

Home now and I dont have the BGB with me,but unless it has changed since the BRB the above statement is not true.
The reg in the BRB that applies if I recall is 411.4.9

If anyone has access to the BGB and can check if 411.4.9 is still there it would clear this up. If 411.4.9 has been removed since the BRB I will stand corrected.

yep....still there...

 

[Deleted member 9648]

I dont stand corrected then!

 

[Darkwood]

Not true.
411.4.9

Direct quote-

The NICEIC advises using Table 41.3 where the installation has been so designed that the overcurrent characteristics of the RCBO to BS EN 61009-1 are being used for automatic disconnection.

A cautionary note must be added here a Zs value that is under 1667Ω only ensures that in the event of an earth fault, the RCD or RCBO should operate. It does not confirm that the circuit is totally compliant with other requirements of BS7671:2008 (2011). For TN-S or TN-C-S systems a Zs value significantly above that required by 41.3 could indicate a high resistance fault somewhere in the circuit which, in turn, could pose a fire risk.

Also, where an RCD is used for automatic disconnection because of high Zs readings, it must be confirmed that the L-N loop impedance is low enough to ensure that the overcurrent protective device operates before thermal damage occurs to the cable.

Yes the regulation does allow such a scenario i must admit but if you challenge any design body or scheme provider the above advice will be similar ..in a TNS or TNCS system using a rcd to satisfy a high Zs value should be done with extreme caution and not as a matter of routine.. again ill stress a circuit with a high Zs on a TN system should be investigated and measures taken to reduce this... suggesting the OP should deliberately introduce a Zs not compliant to Table 41.3 because he has a fall back to table 45.1. is questionable advice.

Don't forget table 45.1 and the regs relating to it was introduced or amended only in the 17th edition and as most of us are old school who design to a previous stricter guideline that is still taught ...

Reg' 411.4.9 gives a buffer zone for when Zs to table 41.3 cannot be achieved (as its been argued its better than nothing hence its introduction) but it should be for good reason and not poor design because utilising this regulation should be a last resort not done by design choice.

We have said from the start ...

-Its a No NO to deliberately introduce a non compliant zs then to fall back on this reg... the first thing would be to try improve the reading to meet table 41.3
-Any circuit new to a TN system should be compliant to table 41.3 regardless -no excuses for poor design, if the Ze is high then account for this in design or enquire to the DNO -if it can't be lowered then yes we have the rcd/rcbo option to fall back on.

If you do an inspection and Ze or Zs is too high you should first establish why and try rectify high reading only then if its not an option and rcd cover isn't installed then you can revert to using this option to satisfy disconnection times ... but ensuring you don't have a fire risk.

Im not saying you cannot use this regulation im saying you shouldn't be applying it loosely to a TN system or questions really have to be asked why you haven't met Zs values in the first place.

 

[Darkwood]

The advice is similar from IET as NICIEC they confirm the use of the regulation but all give caution to using it as a easy option ...RCD's are affected by a number of mains transients and harmonics issues that can blind and mask the sensing coils plus RCD's are not fail safe and have a much larger failure rate by a wide margin compared to mcb's... these are just a few reasons to ensure where possible that Zs requirements are always met to 41.3 with regards to this discussion.

Im probably a bit ahrd minded about this regulation because it has caused quite a stir when introduced and had months of backlashes but at the end of the day its introduction comes down to the consumer is safer where installation don't meet regulations due to nothing more than poor design and incompetence of the installer and as this is becoming even more of an issue with DI wonder courses its implementation into the regs is to back up poor design where it should never have been needed in the first place.

In essence on a TN system this regulation was introduced as a plaster to repair an already bad wound but safety of consumer has to come first regardless of the reasons it exists.

 

[spark 68]

For TN-S or TN-C-S systems a Zs value significantly above that required by 41.3 could indicate a high resistance fault somewhere in the circuit which, in turn, could pose a fire risk.

Hi DW, that is (the bit colored black) is a different scenario where the Zs is marginally higher for the OCPD in question, and more to the point where it is totally expected by virtue of the correct (R1+R2) + Ze, and not a faulty circuit.

I agree with you in that I would not design a new circuit that way, unless as you say due to a unavoidably high Ze where you have no control.

Provided of course that the cable meets all the other requirements of BS7671 (ie. L-N loop, PFC etc.)

 

[Deleted member 9648]

This debate is NOT about using an RCD on a TN system to get over poor design or a fault on a circuit causing a high Zs. The circuit the OP has meets the requirements for a type B,and presumably has a Zs reading which would be expected for the circuit.All he wants to do is use an OCPD which will slightly exceed the max Zs for disconnection within 0.4s.
This debate has come up before and at no point have I ever suggested an RCD could be used to get over a poorly designed or faulty circuit.

 

[Darkwood]

Home now and I dont have the BGB with me,but unless it has changed since the BRB the above statement is not true.
The reg in the BRB that applies if I recall is 411.4.9

If anyone has access to the BGB and can check if 411.4.9 is still there it would clear this up. If 411.4.9 has been removed since the BRB I will stand corrected.

My bad Wirepuller was badly phrased when i printed-

Like we have been saying throughout this thread the 1667 limit is applicable to TT only and if your on a tn system then your rcbo has to meet requirements of table 41.3.

I meant to come across that you should always design a TN system to 41.3 the 'has to' was meant to say 'Should'

 

[spark 68]

TBF I wanted to know more about the VD theory in post #30 with RCDs and RCBOs on TN systems

I have difficulty with understanding that statement.

I would agree with DW that if you initially supplied the RCD/RCBO with a lower than specified Voltage that it may not operate in the expected way.

Assuming that the supply is normal to start with, and neglecting the test button/resistor arrangement for the moment, we know that all the conductors in the circuit have some resistance however low.

If we had a bolted L-E fault with negligible impedance the voltage would attempt to drop across the various resistances in the circuit, while this voltage is attempting to fall the current would also be rising proportionately at the same time, this has to happen by virtue of ohms law, this out of balance current and magnetic field through the RCD would surely have to trip the device ?

 

[Deleted member 9648]

My bad Wirepuller was badly phrased when i printed-

Like we have been saying throughout this thread the 1667 limit is applicable to TT only and if your on a tn system then your rcbo has to meet requirements of table 41.3.

I meant to come across that you should always design a TN system to 41.3 the 'has to' was meant to say 'Should'

Thanks for clarifying.....although the part that I take issue with is that the 1667 limit is applicable to TT only. Not according to 411.4.9 it isnt.That said at no point would I ever suggest that such values should be accepted on a TN. Clearly R1+R2 + Ze applies and Zs readings way off the expected would not be acceptable. But bear in mind that the max Zs for a 32a type C is 0.72....and for a type D 0.36. There are inumerable perfectly healthy TN fed RFC's which exceed these values and to which high inrush current equipment is required to be connected.

 

[spark 68]

Just to throw another grenade into this debate,

Something which DW alluded to earlier, although not specific or exclusive to TN systems, is the fact that certain items of equipment can "blind" the RCD, DC imposed on the lines for example, this saturates the magnetic core so it effectively cannot see the imbalance due to a fault, this is how the old D-Lok no-trip ELI meter works, you are then reliant on the OC part of the RCBO, which is another reason to aim as close as possible to the required Zs.

The RCD is no magic bullet, it may only mitigate a certain risk, and may be unreliable.

I once worked for a panel building company building a large number of DBs, (I was testing for them) the schedules for these various DBs used quite a few RCBOs, several hundred in fact, about 1 in 5 never met the full range RCD tests, either not meeting the times, or not tripping at all, some not turning on with no power applied. These were brand new out of the pack, and were made by a well known manufacturer, so not a cheap no-mark brand.

 

[Deleted member 9648]

In some ways,while this is a good debate, it has gone a little off piste. As far as I'm concerned the OP's question is simply can an RCD be used to meet disconnection times where the OCPD wont on a TN system. The answer,regardless of the technicalities,is that yes it can in full compliance with Bs7671. 411.4.9 applies.
The question of whether it should be permitted is another matter entirely,and is where this thread has gone....I'll leave that to those with more technical knowlege than myself.

 

[Darkwood]

Sorry for the disappearance it was that Valentines crap ... gotta keep her happy but yes Wirepuller we have gone off on a large tangent here so good point to end it maybe for another time and debate closer to the direction the thread went in.

 

[Engineer54]

The RCD is no magic bullet, it may only mitigate a certain risk, and may be unreliable.

I once worked for a panel building company building a large number of DBs, (I was testing for them) the schedules for these various DBs used quite a few RCBOs, several hundred in fact, about 1 in 5 never met the full range RCD tests, either not meeting the times, or not tripping at all, some not turning on with no power applied. These were brand new out of the pack, and were made by a well known manufacturer, so not a cheap no-mark brand.

No need to convince me about the reliability of RCD devices, and as you have found, only become noticeable when you are dealing with large numbers, rather than the one or two, or a couple more for most Domestic/small commercial installations. On projects the size i'm used to dealing with, we are talking in the thousands of RCD devices, and of all types and sizes. Like yours, ours are also from top end manufacturers....

 

[Engineer54]

Sorry for the disappearance it was that Valentines crap ... gotta keep her happy but yes Wirepuller we have gone off on a large tangent here so good point to end it maybe for another time and debate closer to the direction the thread went in.

You Romantic old so and so!! lol!!