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acvc

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OK sorry I'm being really impatient and possibly thick here, but I've just done this for the first time today .....


if a switchfuse is used on tails over 3m long to protect the DNO fuse, should the fuse rating in the switchfuse be less than the DNO main fuse? If not why not? I think I'm missing the point here??????
 

topquark

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OK sorry I'm being really impatient and possibly thick here, but I've just done this for the first time today .....


if a switchfuse is used on tails over 3m long to protect the DNO fuse, should the fuse rating in the switchfuse be less than the DNO main fuse? If not why not? I think I'm missing the point here??????
The switch fuse is to protect the run of cable, not to "protect" the DNO fuse. The DNO suggest that there fuse is only to protect a maximum tail run of 3M, anything over that and you should provide your own protection. The fuse rating should be less than or equal to the DNO fuse for common sense reasons (protecting the DNO fuse, there I've said it!).
 

acvc

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  • #3
Thanks Topquark, but if the two fuses are of the same rating e.g. both 80A or 100A, then which will blow first? Shouldn't there be some discrimination?
 
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1shortcircuit

What size fuse is currently fitted by the DNO and what size cable will you be protecting?

What is the max demand of the installation that is being supplied?

:thumbsup
 

acvc

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7m of 16mm single-core tails, clipped direct, (backwards from C/U) to 80A switchfuse, to DP isolator to meter to 80A main cut out fuse. New mains head +PME+isolator being fitted by DNO tomorrow to this spec.
 

acvc

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Is this issue of discrimination between a switchfuse and main cut-out fuse a non issue?

If they are both e.g. 80A and both HRC, will the fuse closest to the point of fault blow first?
 
You will only achieve discrimination if you make your fuse size significantly lower than that provided by the DNO, and this may not then be adequate for your installation needs.
As has already been said, protect your installation with the correct size fuse, just as long as it is not larger than the DNO one.
 

PEG

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ALL fuses are created equal,but some are more equal than others.......:yinyang:
 

topquark

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If you want to provide some discrimination then put a lower rated fuse in (in correleation with the demand). DNO around here only will offer 60A or 100A now. So use a 100A DNO fuse with maybe an 80A in the switch fuse.

With fuses discrimination is difficult (at best) to achieve with equal fuse sizes. It would be "touch and go" as to which would blow first as they are not exact beasts. Even with the same rating from the same manufacturer.
 
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1shortcircuit

Knowing the max demand of the installation will help you considerably and help you to determine the correct size fuse. Max you can go is 80 which is equal to the DNO but if you could prove max demand will be considerably less then you may be able to prove that a 60A will do the job comfortably.

What installation is it?

I thought that a new install would have been 100A fuse with 25mm tails???
 

telectrix

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it could be argued that a better solution would be to fit a MCB instead of a fuse. as a rule, the MCB would operate well before the DNO fuse, irrespective of discrimination in ratings.
 
M

Mark_Burgess

it could be argued that a better solution would be to fit a MCB instead of a fuse. as a rule, the MCB would operate well before the DNO fuse, irrespective of discrimination in ratings.
Thats the route I'd go down.
 

darkwood

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On industrial it is often the case (example) that say DNO have 200Amps and busbar main isolator for instance might be the same but if you reference the type that DNO use they tend to give allowance i.e. type 3, 4 or 5 against your bs88 type 2 this give your fuses priority, where as some commercial and domestic use standard type 2 DNO fuses and even with reduction of your fuse size in the case of a short circuit full discrimination wont be achieved anyway only partial in some cases..... its usually accepted that domestic cannot achieve full discrim' but to check if 2 fuses up and downstream of each other give full discrimination then check that the total let through energy (I^2t) of the downstream device is less than the pre-arcing energy (I^2t) of the upstream device.... if so then you have acheived full discrim'

Example if you have a bs88 63amp fuse Gg1 it will only achieve full discrimination to a 6amp (b) mcb, 10amp - 40amp will give partial discrim' decreasing respectively as your mcb rating increases but no discrim will be acheive between bs88 63amp and a 50amp and obviously a 63amp mcb type b.

Hope you can appreciate now how hard it is to get full discrimination on short circuit and as my example shows if your front end is a bs88 Gg1 63amp fuse then anything above 6amp (b) mcb wont acheive full discrim'.

Discrimination for overload current is an easier thing to understand and meeting full discrim' in this case isn't rocket science so wont bother explaining that one ;)

............
Ill just add to this (edit)... when discriminating against overload my comment just then is in ref' to similar devices but if you are comparing 2 differing divices a trick is to print the time current curve of one device on paper the the second device on tracing paper and overlay to see what needs to be done to achieve discrim' under overload condition as sourcing comparison charts cn be a pain.

re' short circuit discrim', Hagar's fuseboards catalogue has a tech' section that will compare in chart form different devices from bs88 to mccb's etc etc very helpful reference an im sure you'll be able to get it online.
 
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sparks1973

well...on the basis that an avarage sized house...2-3 beds....may get up to what?...say 60-65A at max demand times....5-7 PM ish....i reckon a 60A will hold nicely for the duration of max demand at least....so if this sounds like its about right for the scenario you have posted about.....
 
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sparks1973

i mean whats this notion that just because the DNO bullit is rated at 100A....that means the property it feeds is gonna pull 100A?..
 
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