Just for curiosity, assuming there were three consumer units all independently fed using different phases. If you were working on a circuit on one of the consumer units and you flicked the mcb for that circuit off (Not the isloator!! ) (thus cutting the live feed to that circuit !) whilst working on it , although the neutral of that circuit would still be connected , is it possible to have a shock from the neutral, as technically it is connected to the other 2 phases , and they are still live to the other consumer units.
3 phase question
- Thread starter jondiy
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only if the neutral is broken before the split point. pretty much the same as when you flick off just one MCB in a single phase setup. as long as the incoming neutral is intact its fine. if it breaks then you can get nasty potentials after the break.
So at the actual split (ie where the henley block for neutrals would divert off to other consumers) there is a high potential , but as long as the incomer to it is left intact , then all neutrals after it are free of potentials, and joining into that block would be ok, provided that the incomer was intact??
For example, if adding another consumer unit into your red phase to run alongside an existing one , provided the red phase fuse had been pulled by the dno, it would be safe to connect your new consumer live to your red phase live, and when connecting the new neutral for new consumer to the current henley blocked neutral , it would be safe to do so, provided the incomer neutral (from the meter) was still connected to that block ? Am I right in this thinking plugwash , or is it only safe once the neutral tails come away from the block (in this case would disconnecting all 3 phase fuses solve that problem?). Its one of those things thats baffled me for years, but luckily as I'm not an electrician, and more of a diy person, I don't really NEED to know - but would LIKE to know for reference sake.
For example, if adding another consumer unit into your red phase to run alongside an existing one , provided the red phase fuse had been pulled by the dno, it would be safe to connect your new consumer live to your red phase live, and when connecting the new neutral for new consumer to the current henley blocked neutral , it would be safe to do so, provided the incomer neutral (from the meter) was still connected to that block ? Am I right in this thinking plugwash , or is it only safe once the neutral tails come away from the block (in this case would disconnecting all 3 phase fuses solve that problem?). Its one of those things thats baffled me for years, but luckily as I'm not an electrician, and more of a diy person, I don't really NEED to know - but would LIKE to know for reference sake.
Your first assumption was correct.
Remove red fuse, and although yellow and blue remain live, as long as the neutrals are tied to neutral, they will remain at (or near) 0v.
Remove red fuse, and although yellow and blue remain live, as long as the neutrals are tied to neutral, they will remain at (or near) 0v.
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lookinn said:
yes neutral disconnection on a PME is bad.
btw lost neutral on a 3 phase system can cause all sorts of problems. It won't simply stop working like with single phase. instead the potential of the lost neutral will jump all over the place.
plugwash said:
Yea, with one phase receiving more than 240 (and upto 415), and another receiving proportionally less.....eek!
Thanks guys for the info.
With reference to Crafty's points of a TP&N distribution board. But you CAN still have seperate CU's running from a TP&N Distribution board - So no need to get rid of them! Yes, the actual place I was referring to does have a TP&N DB, and obviously with its TP&N isolator, then its easier to work on! , However the reason this installation has seperate CU's is because it is a multi storey building, it makes it easier to work on different floors of the building without having to keep going back to the meter room to isolate independent MCB's. There are 3 consumers already fed from 60A MCB's on the DB, and the rest of the MCB's on the DB are all full of smaller circuits ( mostly local to the meter room and DB) . As the board uses Crabtree C50's , the 60 Amp mcb's are now virtually obsolete , hence looking at the option of adding a Switchfuse (using tails from current splits) to feed a new Consumer in a location (more than 3 meters away!) . Have checked the phase loads with a clamp meter with all circuits maxxed out! and theres plenty of spare current available on all the phases, so not worried about overloads!
Seen as the tails have already been split into Henley blocks, it makes it just as easy as if I had gotten hold of another 60A MCB!
Other than that its a whole new modern distribution board - and a lot of time to swap all the wiring over! - Compared to the addition of a simple Switchfuse and a couple of tails! The latter seems the most cost effective method , especially as I'm the one who'd be footing the bill for it all, and it seems a shame to ditch that Crabtree DB - its been working fine for 20+ years, and never had to replace an MBC in it so far!
Opinions from anyone out there much appreciated. As long as the loads have all been balanced out on the phases, I can't see any harm in the above method of connection!
By the way, please don't quote me 'Notification Required!!' , Building Regulations , Part P etc , I have'nt come on here for a lecture, I am just looking for advice!
With reference to Crafty's points of a TP&N distribution board. But you CAN still have seperate CU's running from a TP&N Distribution board - So no need to get rid of them! Yes, the actual place I was referring to does have a TP&N DB, and obviously with its TP&N isolator, then its easier to work on! , However the reason this installation has seperate CU's is because it is a multi storey building, it makes it easier to work on different floors of the building without having to keep going back to the meter room to isolate independent MCB's. There are 3 consumers already fed from 60A MCB's on the DB, and the rest of the MCB's on the DB are all full of smaller circuits ( mostly local to the meter room and DB) . As the board uses Crabtree C50's , the 60 Amp mcb's are now virtually obsolete , hence looking at the option of adding a Switchfuse (using tails from current splits) to feed a new Consumer in a location (more than 3 meters away!) . Have checked the phase loads with a clamp meter with all circuits maxxed out! and theres plenty of spare current available on all the phases, so not worried about overloads!
Seen as the tails have already been split into Henley blocks, it makes it just as easy as if I had gotten hold of another 60A MCB!
Other than that its a whole new modern distribution board - and a lot of time to swap all the wiring over! - Compared to the addition of a simple Switchfuse and a couple of tails! The latter seems the most cost effective method , especially as I'm the one who'd be footing the bill for it all, and it seems a shame to ditch that Crabtree DB - its been working fine for 20+ years, and never had to replace an MBC in it so far!
Opinions from anyone out there much appreciated. As long as the loads have all been balanced out on the phases, I can't see any harm in the above method of connection!
By the way, please don't quote me 'Notification Required!!' , Building Regulations , Part P etc , I have'nt come on here for a lecture, I am just looking for advice!
Well at least a DB looks neater than this:
(supply to 6 flats and a carpark/communal area) Note each phase has 2 x 100A fuses on it, with the exception of the leftmost phase on the service head which has THREE 100A fuses, one of which supplies that meter and a length of 2.5mm² cable which at the time it was found was supplying about 29A
Think they could have done with a DB when they installed this lot, supplying the 7 meters from there.
(supply to 6 flats and a carpark/communal area) Note each phase has 2 x 100A fuses on it, with the exception of the leftmost phase on the service head which has THREE 100A fuses, one of which supplies that meter and a length of 2.5mm² cable which at the time it was found was supplying about 29A
Think they could have done with a DB when they installed this lot, supplying the 7 meters from there.
jondiy said:
Who would be doing this work?jondiy said:
Who would be adding it?
Who checked it?
The only reason you should fear a lecture is if you know you are doing things that you shouldn't. Nobody here is going to refuse advice that will keep you safe, but reminding you of the law (if it applies) is not a bad thing, nor is it a "lecture".
I for one would be interested to know what this building is, with its multiple floors, and multiple CUs, and what your involvement is with it, and with the electrical installation...
You can get a shock off the neutral if the 3 phases are not balanced. Also if there is a high resistance supplying the property.
TeesdaleSpark said:
Just trying to get my head around how on a PME installation you can get a shock from a connected neutral?
If the neutral & earth are connected together how can one rise in potential and one not?
I'm sure someones going to tell me
If you touch a neutral conductor and also are touching earth you are making a circuit. There is a difference in the resistances back to the transformer, or pme service head, therefore a potential difference i.e voltage.
Unbalanced 3 phase loads induce a current in the neutral making the problem worse.
The following link may help in explaining whats happening in a three phase system
http://www.iee.org/Forums/forum/messageview.cfm?catid=5&threadid=9041
The following is also an interesting article:
http://www.bomara.com/powervar/wp-neutral-ground-voltage.htm
When I was an apprentice an Electrician pulled the fuse on a lighting circuit I was working on. It was in a wet area. I touched the neutral and got a zap.
Unbalanced 3 phase loads induce a current in the neutral making the problem worse.
The following link may help in explaining whats happening in a three phase system
http://www.iee.org/Forums/forum/messageview.cfm?catid=5&threadid=9041
The following is also an interesting article:
http://www.bomara.com/powervar/wp-neutral-ground-voltage.htm
When I was an apprentice an Electrician pulled the fuse on a lighting circuit I was working on. It was in a wet area. I touched the neutral and got a zap.
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