IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Discrimination of MCBs???
Topic Summary:
Created On: 10 February 2010 05:52 PM
Status: Post and Reply
Linear : Threading : Single : Branch
1 2 Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 10 February 2010 05:52 PM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Hi all,

My mate wants to extend his salon to the basement, due to his current demand he will need a 3PH meter put in place of his SPH one. I need to put in a 6 way 3PH DB in place of his current SPH CU and install a new 6 way 3PH DB in the basement to cater for that part. Im also installing a 3PH 63a Switch Fuse. My preferred method for installing the 3PH DB in the basement is to supply it from the new 3PH DB upstairs, using an adequately rated 3PH MCB, but from what I have read before, supplying a DB with a load of MCBs from an MCB isnt a good thing for discrimination when it comes to fault currents. My other option would be to supply both DBs from the same terminals of the switch fuse, but im not happy with two separate conductors being screwed into one terminal because of the fact that they can become loose over time. Im sure there are crimping methods, but I dont have the money for those nice expensive hydraulic crimps. How would the more experienced in this field do it please?

-------------------------
Regards
Carl
 10 February 2010 06:01 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19433
Joined: 23 March 2004

Buy equipment that allows both MCB's and BS 1361/BS 88 devices to be installed (Hager for example). You could then do as you originally planned but use fuse carriers in place of the outgoing MCB.

You could also consider henley blocks with a switch fuse at the origin to maintain single point of isolation whilst feeding both boards from the henley blocks - you probably won't get discromination between the switch fuse and ethe DNO fuses but at that point who cares.

Failing that, who says you need full discrimination in a wash and blow set up does it really matter if the basement DB goes off for a single fault - presumablly you have em/lighting in place

Regards

OMS

-------------------------
Failure is always an option
 10 February 2010 06:30 PM
User is offline View Users Profile Print this message



lyledunn

Posts: 607
Joined: 13 August 2003

Carl,
Just come back from sitting through a long court session relating to the failure of emergency lighting installed by a local contractor. I have been in court many times (as a witness, I hasten to add) and I really shouldnt need reminding that if you decide to ditch the guidance of a recognised authority then you do so at you peril.
From my observation of this forum, you would probably have to go a long way to get better advice than that usually given by OMS but even though the consequences of the lack of discrimination in your case would, on the face of it, seem rather benign, I dont suppose that you will find too many experts to back you in court should some unforeseeable event occur!
By the way, I learned a new word today; "Kafkaesque". It was used by one barrister to describe the slap dash approach of the contractor to matters relating to accepted industry practice!

-------------------------
Regards,

Lyle Dunn
 10 February 2010 07:05 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19433
Joined: 23 March 2004

"Kafkaesque"


LoL - I've had my designs called many things both good and bad Lyle but never "Kafkaesque"

I could have, perhaps, applied it other aspects of my life but there you go

Regards

OMS

-------------------------
Failure is always an option
 10 February 2010 08:09 PM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Originally posted by: lyledunn

Carl,

Just come back from sitting through a long court session relating to the failure of emergency lighting installed by a local contractor. I have been in court many times (as a witness, I hasten to add) and I really shouldnt need reminding that if you decide to ditch the guidance of a recognised authority then you do so at you peril.

From my observation of this forum, you would probably have to go a long way to get better advice than that usually given by OMS but even though the consequences of the lack of discrimination in your case would, on the face of it, seem rather benign, I dont suppose that you will find too many experts to back you in court should some unforeseeable event occur!

By the way, I learned a new word today; "Kafkaesque". It was used by one barrister to describe the slap dash approach of the contractor to matters relating to accepted industry practice!


Thank you Lyle, I take on board what you say. Although I am not a contractor (I work for a large one as a QS), I dont consider myself as slap dash, just wanted experienced replies from all walks of life on how they would discriminate in this circumstance, or perhaps know of a better way than myself. I intend to install fully inline with industry standards. There are emergency lighting already installed, but i will need to install more for my part. I haven't seen the spec for that part of the job yet anyway. Im concentrating on the alteration aspect at the moment from SPH to 3PH and how im going to work with western power on this.

Thanks

-------------------------
Regards
Carl
 10 February 2010 08:10 PM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Originally posted by: OMS

Buy equipment that allows both MCB's and BS 1361/BS 88 devices to be installed (Hager for example). You could then do as you originally planned but use fuse carriers in place of the outgoing MCB.



You could also consider henley blocks with a switch fuse at the origin to maintain single point of isolation whilst feeding both boards from the henley blocks - you probably won't get discromination between the switch fuse and ethe DNO fuses but at that point who cares.



Failing that, who says you need full discrimination in a wash and blow set up does it really matter if the basement DB goes off for a single fault - presumablly you have em/lighting in place



Regards



OMS


Thanks OMS, I will do a little more thinking on the subject.

-------------------------
Regards
Carl
 10 February 2010 08:33 PM
User is offline View Users Profile Print this message



tattyinengland

Posts: 781
Joined: 23 November 2006

Now given who has commented above, and thier respected reputations, I'll stick my head on the chopping block and must admit to seeing nothing wrong with supplying the second DB from the first DB. - Supply the local EM lights from the local circuit, which ever board that circuit comes from.

A 3 phase DB would often supply multiple other 3 phase boards.

Is this considered bad practice???
 10 February 2010 08:51 PM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Originally posted by: tattyinengland

Now given who has commented above, and thier respected reputations, I'll stick my head on the chopping block and must admit to seeing nothing wrong with supplying the second DB from the first DB. - Supply the local EM lights from the local circuit, which ever board that circuit comes from.



A 3 phase DB would often supply multiple other 3 phase boards.



Is this considered bad practice???


The set up at present is a single phase switch fuse with a 60a BS 3036. It supplies a 12 way CU. The fuse blew the other day and all power lost. WPD came out, said it was nothing to do with them really, but changed the fuse wire, it wasn't another 60a, but it will do apparently??? The current was measured at the time and 70a was flowing. He wants to extend the shop with all but the same spec, thats where I told him to upgrade meter etc....

Anyway, the EM LTG is supplied from that one CU, so when I replace for a 3PH board i will obviously keep the EM supply there. EM LTG installations ive seen didnt have a dedicated circuit, they were wired off the existing LTG circuit/s?

Thanks

-------------------------
Regards
Carl
 10 February 2010 09:05 PM
User is offline View Users Profile Print this message



tattyinengland

Posts: 781
Joined: 23 November 2006

I know wylex do a 3 phase rotary switched fused isolator, both 100A and 60A. If I remember correctly this isolator has more than one termination point on the bottom. It's pretty good for SWA too, loads of space to terminate the cables. How about that for a plan?
 10 February 2010 11:34 PM
User is offline View Users Profile Print this message



baldelectrician

Posts: 311
Joined: 11 June 2005

Feeling a bit dim here, but what's wrong with having the new DB at source supplying a remote DB with a type c/d breaker (subject to Z db being ok)?

The new db would be protected by RCBO's (type B) in normal circumstances thus giving discrimination

This would be easier if the new DB's were Merlin / Hager (ie a good make / decent kit)

-------------------------
baldelectrician.com
 11 February 2010 07:05 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19433
Joined: 23 March 2004

Originally posted by: tattyinengland

Now given who has commented above, and thier respected reputations, I'll stick my head on the chopping block and must admit to seeing nothing wrong with supplying the second DB from the first DB. - Supply the local EM lights from the local circuit, which ever board that circuit comes from.

A 3 phase DB would often supply multiple other 3 phase boards.

Is this considered bad practice???


Feeling a bit dim here, but what's wrong with having the new DB at source supplying a remote DB with a type c/d breaker (subject to Z db being ok)?


I suggest you sketch the characteristics of say a 32A Type B MCB onto the characteristics of say a 63A or 80A Type C MCB and then draw a line vertically at say 1kA PSCC and report the impact on both breakers taking into account the instantaneous tripping characteristic

As a general rule, an MCB will not discriminate with an upstream MCB in most circumstances

Regards

OMS

-------------------------
Failure is always an option
 11 February 2010 11:37 AM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Originally posted by: tattyinengland

I know wylex do a 3 phase rotary switched fused isolator, both 100A and 60A. If I remember correctly this isolator has more than one termination point on the bottom. It's pretty good for SWA too, loads of space to terminate the cables. How about that for a plan?


That sounds good tatty, but I've already got a controlgear rotary isolator, quite a large unit too. Didn't expect it to be so big to be honest!!

-------------------------
Regards
Carl
 11 February 2010 11:41 AM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Originally posted by: OMS

Originally posted by: tattyinengland



Now given who has commented above, and thier respected reputations, I'll stick my head on the chopping block and must admit to seeing nothing wrong with supplying the second DB from the first DB. - Supply the local EM lights from the local circuit, which ever board that circuit comes from.



A 3 phase DB would often supply multiple other 3 phase boards.



Is this considered bad practice???




Feeling a bit dim here, but what's wrong with having the new DB at source supplying a remote DB with a type c/d breaker (subject to Z db being ok)?




I suggest you sketch the characteristics of say a 32A Type B MCB onto the characteristics of say a 63A or 80A Type C MCB and then draw a line vertically at say 1kA PSCC and report the impact on both breakers taking into account the instantaneous tripping characteristic



As a general rule, an MCB will not discriminate with an upstream MCB in most circumstances



Regards



OMS


Exactly what I thought and why I have asked for opinions. Looks like I will need to supply both D.Bs from the isolator!

Thanks for the input everybody!!

-------------------------
Regards
Carl
 11 February 2010 04:51 PM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8824
Joined: 03 October 2005

I think you will have to agree that discrimination is far beyond most of this forum and is best left to designers, often it is not feasible especially total discrimination as the level of the fault current that needs to or the time for which the current flows often causes too much stress on the system so experienced designers have to achieve some sort of balance between potential damage and disruptions caused by a fault. (cost

The best you can achieve is partial discrimination and take advantage of 'back up protection' then you can exploit the current/energy limiting qualities of circuit breakers and fuses.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 11 February 2010 05:06 PM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Originally posted by: rocknroll

The best you can achieve is partial discrimination and take advantage of 'back up protection' then you can exploit the current/energy limiting qualities of circuit breakers and fuses.



regards


thanks r n r, what do you mean by this? An example maybe please?

-------------------------
Regards
Carl
 11 February 2010 06:58 PM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8824
Joined: 03 October 2005

The majority of LV installations have partial or no discrimination and this is permitted by the regulations by using 'back up protection' this exploits the current/energy limiting effect of the upstream device that enables you to use smaller breaking capacity devices than you really need.

The fault current the downstream device 'sees' is what the upstream device allows through, hence the 100A 33kA current limiting service fuse and the 6kA energy limiting circuit breakes, the only stipulation is that the upstream device must have a breaking capacity to match the highest possible fault current which it has.

So, however you do the job, henley the tails, insert into the main switch the 'back up fuse' will protect your system whether you have partial or no discrimination, fairly bomb-proof.

Like 'discrimination' the theory behind 'back up protection' is fairly complex but the DNO have done all the work for you in fitting a BS1361/BS88 fuse which limits the amount of fault current into the LV system.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 11 February 2010 at 07:29 PM by rocknroll
 11 February 2010 07:30 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19433
Joined: 23 March 2004

thanks r n r, what do you mean by this? An example maybe please?


As a starting point Carl - try a google for "pre arcing I2t" and "total I2t".

The former is the energy let through of a device in fault conditions before the contacts or fuse elements start to open (it's essentially the point of no return). The total I2t is the total amount of energy through the device under the fault current reaches zero.

In very simple terms, to achieve total discrimination, the total energy let through of the downstream device must be less than the pre arcing I2t of the upstream device. If it isn't - nothing on earth is going to stop that device opening even if the relative total I2t values are matched.

Now think about I2t - its essentially the fault current - which is influenced by resistance - so an example of total, partial and zero discrimination could be a fault close up to a fuse board where discrimination is not achieved - if the fault occurs along the sub main you may reach a point where the fault level is low enough for disrcrimination to be achieved (ie it's partial depending onm the fault location) and then for a fault at the sub main end the fault current is depressed further and total discrimination is achieved.

Th edesigner can then determine how likley the close up fault is and choose to ignore it - ie he has a partially discriminating system

Back up protection is slightly different insofar as it relies on the energy limiting characteristics of an upstream device to protect a down stream device of lower breaking capacity.

The both concepts are linked however by the idea of pre arcing and total I2t which is often less than the prospective fault currents predicted - as my learned friend pointed out

Regards

OMS

-------------------------
Failure is always an option
 11 February 2010 07:41 PM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8824
Joined: 03 October 2005

Try the right post: LOL

as my learned friend pointed out


Thankyou Grasshopper, for your kind words, the cheque is in the post.

To infinity and beyond...............................

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 11 February 2010 07:53 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19433
Joined: 23 March 2004

To infinity and beyond...............................


Far be it from me to nit pick O wise one - but can you actually go beyond infinity ?

What's on the other side of it - as it were

Regards

OMS

-------------------------
Failure is always an option
 11 February 2010 08:12 PM
User is offline View Users Profile Print this message



CarlCosby

Posts: 484
Joined: 16 March 2009

Thanks for the advise.

Talking about infinity, did anybody see horizon last night where infinity was the subject? It did my head in to be honest, I will never think of the word infinity the same way again!

BBC iPlayer if you missed it.

-------------------------
Regards
Carl
IET » Wiring and the regulations » Discrimination of MCBs???

1 2 Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



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