installing a sub board

[Darkwood]

This set up will only give load discrimination and not short circuit, if you intend to supply a submains which has spare ways you have to account that other loads are added, if your doing a dedicated board and have discussed this option with client then you need to know what your largest mcb rating is and find a comparison chart to a BS88 fuse that will give full discrimination, where this means your fuse is higher than the DNO cutout then there is little you can do but to cover with the largest BS88 you can fit regarding the KVA of the supply and give partial discrimination.

You seem to be trying to design without taking into account for discrimination of fusing, this can be costly to the end user if a fault occurs on a final circuit as there is no gaurantee that the final circuit mcb would trip before the 40amp Bs88 you propose.

Also why are you giving 4pole isolation?.. you will be fine with 3pole, if you do 4pole which is not required then ensure any device has early make/late break N operation or you could blow any electronics and control gear in the machinery (dependant on the machine) and any 1ph equipment could suffer damage too just with the action on switching a 4 pole device that covers multiple circuits.
You only require 4pole Isolation at the origin if it is highly likely that the untrained end user or other person may be using it for isolation purposes, this is why domestic has to have both L/N isolated because untrained ppl use the switch to turn the power off.

 

[electrical19]

Be aware of the need to separate outlets on different phases where possible, ie group each phase outlets together instead of alternating. Where the two outlets are within reach on different phases label with a warning of 400V.
Label all outlets, including phase, as I have seen too many commercial / industrial works with missing or incorrect labeling.

Yes you are perfectly right.
I seen the worse
Last time I seen the light switch with 2 switches 400v .
How can people do these type of mistake

 

[Darkwood]

Be aware of the need to separate outlets on different phases where possible, ie group each phase outlets together instead of alternating. Where the two outlets are within reach on different phases label with a warning of 400V.
Label all outlets, including phase, as I have seen too many commercial / industrial works with missing or incorrect labeling.

As far as I'm aware this clause was dropped from the regs and also the clause that required say multigang switching that contains more than 1ph used to require labelling with 400v but its been revised to anything with voltages in excess of 230v to earth or where voltage ratings are unexpectedly higher for said accessory or enclosure. At the end of the day, anyone open these points should be qualified and following all safety isolation and testing procedures.

 

[Darkwood]

Yes you are perfectly right.
I seen the worse
Last time I seen the light switch with 2 switches 400v .
How can people do these type of mistake

As my above post implies, there is nothing wrong with having different phases behind multigang switches, its common in commercial and Industrial and is how you acheive balance on phases if running loads direct through the switching.. many domestic installers perceive this to be bad practice or even against the regs - innexperience with larger commercial and Industrial jobs is probably a factor to this misconception but it is widely used practice and done by myself on a few occasions.

 

[telectrix]

any electrician with a brain will realise that if premises have a 3 phase supply, there is possibility of all 3 phases being present in an enclosure. common sense leads to h&s.

 

[electrical19]

This set up will only give load discrimination and not short circuit, if you intend to supply a submains which has spare ways you have to account that other loads are added, if your doing a dedicated board and have discussed this option with client then you need to know what your largest mcb rating is and find a comparison chart to a BS88 fuse that will give full discrimination, where this means your fuse is higher than the DNO cutout then there is little you can do but to cover with the largest BS88 you can fit regarding the KVA of the supply and give partial discrimination.

You seem to be trying to design without taking into account for discrimination of fusing, this can be costly to the end user if a fault occurs on a final circuit as there is no gaurantee that the final circuit mcb would trip before the 40amp Bs88 you propose.

Also why are you giving 4pole isolation?.. you will be fine with 3pole, if you do 4pole which is not required then ensure any device has early make/late break N operation or you could blow any electronics and control gear in the machinery (dependant on the machine) and any 1ph equipment could suffer damage too just with the action on switching a 4 pole device that covers multiple circuits.
You only require 4pole Isolation at the origin if it is highly likely that the untrained end user or other person may be using it for isolation purposes, this is why domestic has to have both L/N isolated because untrained ppl use the switch to turn the power off.

Thank you
The 40 amp will be an mcb type c
The gg fuses are at the intake to protect the DB my idea is to get power from the DB
Fit 40amp mcb and connect the swa to the subboard.

 

[Darkwood]

Thank you
The 40 amp will be an mcb type c
The gg fuses are at the intake to protect the DB my idea is to get power from the DB
Fit 40amp mcb and connect the swa to the subboard.

You should not run a submain of a mcb from another DB, unless you have a power board 'MCCB's' or similar busbar set-up which will usually have larger incomming to allow for all the submains then I would suggest you tail up a 3ph isolator with BS88 II (80amps) in design your cable to this rating and at least you have partial discrimination if the supply cutout is only 100amp..

You repeatedly seem to be missing my point regarding discrimination, if your customer is happy that he may on occasion lose the entire DB and all the machinery on it to ensure a budget install then fine, poor design, poor dicrimination but get it in writing and confirmed by your customer then when he has lost half a day with a fault and no machines running then at least you covered your back and he cannot claim against you. I promise you the next sparky to come along and help him sort out why the wrong fuses are going will have no qualms about expressing how badly designed the install was.

If your going to be doing this kind of work then it may benefit you to take a design course which will go someway to helping you understand the very different requirements, methods and knowledge of large commercial and industrial require, going into this side with domestic experience only can be very costly as its a whole different ball game.

Don't get me wrong, I'm trying to give constructive advice here as your responses give the impression you a little out of your depth, we haven't even asked about the machinery and wht they do (although at that low demand I suspect it not relevent), this also can reflect on the final circuit OCPD which may need to allow for inrush of say motors etc.

Your 40amp (c) mcb on a cable rated for load and volts drop will work fine when all is fine and dandy but your design comes into its own when you get a fault and the correct OCPD operates and not one further upstream.

 

[davesparks]

Be aware of the need to separate outlets on different phases where possible, ie group each phase outlets together instead of alternating. Where the two outlets are within reach on different phases label with a warning of 400V.
Label all outlets, including phase, as I have seen too many commercial / industrial works with missing or incorrect labeling.

Why? It is not a requirement of the current regulations, not was it of the previous edition.

A voltage warning label is only required where the voltage to earth exceeds 230V nominal. All normal 400/230 3ph supplies do not require such labels.

 

[electrical19]

You should not run a submain of a mcb from another DB, unless you have a power board 'MCCB's' or similar busbar set-up which will usually have larger incomming to allow for all the submains then I would suggest you tail up a 3ph isolator with BS88 II (80amps) in design your cable to this rating and at least you have partial discrimination if the supply cutout is only 100amp..

You repeatedly seem to be missing my point regarding discrimination, if your customer is happy that he may on occasion lose the entire DB and all the machinery on it to ensure a budget install then fine, poor design, poor dicrimination but get it in writing and confirmed by your customer then when he has lost half a day with a fault and no machines running then at least you covered your back and he cannot claim against you. I promise you the next sparky to come along and help him sort out why the wrong fuses are going will have no qualms about expressing how badly designed the install was.

If your going to be doing this kind of work then it may benefit you to take a design course which will go someway to helping you understand the very different requirements, methods and knowledge of large commercial and industrial require, going into this side with domestic experience only can be very costly as its a whole different ball game.

Don't get me wrong, I'm trying to give constructive advice here as your responses give the impression you a little out of your depth, we haven't even asked about the machinery and wht they do (although at that low demand I suspect it not relevent), this also can reflect on the final circuit OCPD which may need to allow for inrush of say motors etc.

Your 40amp (c) mcb on a cable rated for load and volts drop will work fine when all is fine and dandy but your design comes into its own when you get a fault and the correct OCPD operates and not one further upstream.

You are perfectly right.
I do not have much experience on the commercial side.
Now I need to know what you mean when you say Discrimination (please don't get me wrong I know what discrimination means in terms of electric work). I need your expertise .
My subboard will be protected by an mcb three phase 40 amp .
The subboard will have a 4 pole main switch rated at 63amp.
Each equipment will have a dedicate radial circuit protected by an rcbo16amp type c. Is this not discrimination?
With regard the Pscc once tested my fluke should read if the mcb ot rcbo will get the bang without explode.by the way thank you your explanation is a gift and helps electricians to improve their knolewge.

 

[Darkwood]

You are perfectly right.
I do not have much experience on the commercial side.
Now I need to know what you mean when you say Discrimination (please don't get me wrong I know what discrimination means in terms of electric work). I need your expertise .
My subboard will be protected by an mcb three phase 40 amp .
The subboard will have a 4 pole main switch rated at 63amp.
Each equipment will have a dedicate radial circuit protected by an rcbo16amp type c. Is this not discrimination?
With regard the Pscc once tested my fluke should read if the mcb ot rcbo will get the bang without explode.by the way thank you your explanation is a gift and helps electricians to improve their knolewge.

Im heading out now but really think you need to get your notes out and read up on what you were taught about discrimination, we are not talking load discrimination but short circuit discrimination here, in a fault situe' as you propose your 40amp(c) is just as likely to trip as any of your 16amp (c) as they have very little partial discrimination and maybe only 2 a few hundred amps, as your PSSC will probably be 1000amps plus then its just random to which device if not both see the fault first.

I'm stopping short on purpose of designing the install as this isn't what you should be using the forum for but I'm guiding you to do your own research in the right area so you can come back with a designed install that we can say 'that will be fine'... again forget about taking this submain off this board unless you want to have issues with upstream tripping of the submains protection due to final circuit faults.

 

[Leesparkykent]

Discrimination is the selection of protective devices so that the device nearest to a fault will operate rather than any upstream device. The purpose is to ensure that the fault is isolated and supply is maintained to other parts of the installation without disruption.

Fuses are frequently used in conjunction with circuit-breakers because they have a higher short-circuit breaking capacity.

Manufacturers produce time/current curves for their fuses and circuit-breakers. Curves are also produced in Appendix 3 of BS7671, where they are based on the slowest operating times for compliance.

By comparing the curves for the two devices, it is possible to determine whether full overload discrimination is achieved or, if not, at what levels of overload current it is achieved.


Short circuit discrimination involves comparison of the total let-through energy and pre-arcing energy for the prospective fault level concerned. Discrimination is achieved if the total let-through energy of the circuit-breaker is less than the pre-arcing energy of the fuse, at all values of fault level.

Fuse manufacturers produce tables listing the pre-arcing energy of their devices while circuit-breaker manufacturers produce curves presenting the total let-through energy at different values of prospective fault current.

 

[GMES]

I am getting quite concerned by what I am reading within this thread, This is not a pop at you Electrical19 but if you don't know what discrimination is and what it involves then you really shouldn't be doing this type of work. I would suggest you find someone with the relevant experience needed to assist you with this job.

I would also urge members to be cautious about handing out a step by step guide of how to do this job.

 

[GMES]

After much thought I feel that it is wrong to give a step by step guide of how to do the job so I am closing the thread.
Electrical19 don't take it personally we just can't allow members to design the install for you but please stick around feel free to engage in other threads.

 

[GMES]

Thread closed.

 

[Darkwood]

The op has now been given enough incite here into the flaws of his design, it's at a good point where the op has received more than enough direction here and I'm in total agreement with GMES that he needs to think seriously about taking on such work when it's clear there is a struggle to understand how to design the install with discrimination as a key point which is a key basic knowledge in industrial..

Thread closed ... hopefully the OP will have a good think about taking on such work.

Electrical 19 ...please take this as a constructive move and hopefully you can take a little bit of a learning curve as to the more complex level of designing for industrial.