I heard somewhere that when wiring a consumer unit, the highest amp breaker should go next to the main switch and lowest amp farther down the board. Is this correct? I have an old Wylex board with rewireable fuses fitted, the fuses are fitted in no particular order and the highest rated fuse is 30a. I got some replacement mcb's and wondered if I should just replace in the same slot or move all the fuses so they are in order. Thanks in advance for any advice [/list]
Placement of mcb's in my CU
- Thread starter filthyturk
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How old?
Piccy?
Piccy?
Why?
If you have got MCBs to replace fuses because they are frequently blowing then further investigation is required as to the cause.
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Regardless of anything else the correct rating fuse/mcb MUST be attached to the correct circuit.
In which case changing the order would require re-wiring the fusebox in order to re-order the circuits and hence breakers. Although its mildly prefrable to have the larger power circuits near the main switch if the current system works and you are simply replacing the fuses with plugin mcbs I would not reorder or have someone reorder the circuits and instead simply swap fuses for mcbs in a like for like fashion.
The others are correct to question why you are replacing the fuses, but I will leave it to them to feild these questions as Im not an electrican and its somewhat up for debate. Obviously its better to replace the whole unit, but more costly. Simply replacing the fuses with MCBs will not give you RCD protection, which all modern consumer units have.
Daniel
In which case changing the order would require re-wiring the fusebox in order to re-order the circuits and hence breakers. Although its mildly prefrable to have the larger power circuits near the main switch if the current system works and you are simply replacing the fuses with plugin mcbs I would not reorder or have someone reorder the circuits and instead simply swap fuses for mcbs in a like for like fashion.
The others are correct to question why you are replacing the fuses, but I will leave it to them to feild these questions as Im not an electrican and its somewhat up for debate. Obviously its better to replace the whole unit, but more costly. Simply replacing the fuses with MCBs will not give you RCD protection, which all modern consumer units have.
Daniel
As far as I can tell the board is fine, no problems at all. I got the mcb's purely for the convenience of just flipping the switch if there is a short and not having to fiddle and fanny about with fusewire. Also my in-laws have the same board and it caught fire! Have posted pics for those that want to see the board, apologies for poor quality light, there's no battenholder under the stairs at the mo View media item 42266 View media item 42270
Whatever was the cause of that it was not because it had fuses instead of MCBs.
K
kai
It is considered good engineering practice to place the largest fuses adjacent the Main Switch, and the others in descending order along the busbar, with the smallest circuits out furthest along the busbar.
Whilst I know it always seems to be considered "good practice", can anyone suggest a valid engineering reason ? It's not like the busbar(s) is of limited CSA or liable to create excessive volt drops 
I had reason to work in the CU in a mates workshop - the complete and utter dickheads that installed the supply for his vehicle lift took the trouble to move all the MCBs down one to fit theirs next to the switch, but also failed to relabel any of them, or to similarly move either neutrals or earths
Given the CGA and length of the bar (especially compared with the length of both supply and load cables), I can't see it making one sh**e of difference.
I had reason to work in the CU in a mates workshop - the complete and utter dickheads that installed the supply for his vehicle lift took the trouble to move all the MCBs down one to fit theirs next to the switch, but also failed to relabel any of them, or to similarly move either neutrals or earths
Given the CGA and length of the bar (especially compared with the length of both supply and load cables), I can't see it making one sh**e of difference.
As you imply, this 'good practice guidance' probably originated from someone who understood the concepts qualitatively, but not quantitatively. In fact, I would have thought that a more relevant (than voltage drop) consideration would be that of heat generated within the MCBs when heavily loaded - and that would best be addressed by advising that one should not put high-rated MCBs next to one another (but, rather, should put low-rated ones in between them), wouldn't it?Whilst I know it always seems to be considered "good practice", can anyone suggest a valid engineering reason ? It's not like the busbar(s) is of limited CSA or liable to create excessive volt drops
Kind Regards, John.
Like others have mentioned it is considered to be good practice and is how I was trained.
But no requirements suggest that this should be done in that way!
It's always worth stopping to question some of these notions of 'good practice' which get passed on from generation to generation (teachers to students, who go on to become teachers ....!!).
As in my previous post, if I asked you whether you thought it was better practice to install (potentially) higher heat-producing (and to some extent heat-sensitive) items in contact side-by-side, or to separate them by lower heat-producing ones, what would you say?
Kind Regards, John.
Some older wooden framed Wylex boards had a 60A main switch and only 1 way capable of taking upto 45A fuses.
The fuse carriers were shaped in such a way to prevent carriers larger than 30A fitting
Some older MEM ones were like this also
The fuse carriers were shaped in such a way to prevent carriers larger than 30A fitting
Some older MEM ones were like this also
Indeed so, and that one position that could take >30A fuse carrier was adajacent to the integral main switch - but why? They were beautifully engineered boards (I particularly liked the concept of two screws for each cable termination) with pretty meaty busbars, and anyone capable of fairly basic electrical calculations should have realised that voltage drop/heating was not got to be an issue in the busbar.
Mind you, although well-intentioned, Wylex essentially only had control over the colour of dots on fuse carriers (and colour of shrouds) in certain positions. What size fuse wire people put into them was beyond their control
Kind Regards, John.
I would say, that method of instalment could well make sense.
But it makes you wonder why when you buy fully loaded boards, that the MCBs that are pre-installed. From my experience start with the higher amp device at the isolator. Whether it be the main or RCD.
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