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MG Midget and Sprite Technical - Fuses

I am about to rewire my Frog and will add a few extra fuses where appropriate. This concerns me:

" According to modern standards, a fuse's rating is the maximum current it can reliably carry without "fusing" (i.e., open-circuiting or "blowing"). Previously, other standards were used; for example, in England, at the time the Sprite was built, fuses were rated as the the current that would cause "instantaneous" fusing. As we shall see, such a current is difficult to define precisely. Typically, a fuse is assumed to blow "instantly" at approximately twice its rating. " (quote from http://www.nonlintec.com/sprite/electrical/)

So, if my understanding of this is correct, the fuses in the original fuse box were marked 35A which would have meant "failing at 35A"

There are a variety of 30mm glass fuses available. Is there a way of knowing (as the spec rarely says) whether the rating on the fuse is "old style" failure current, or new style normal operating current? Since the difference is double the current, getting it wrong could mean that the fuse isn't going to offer the protection needed.

(I would have thought the the 35A fuse "failing" at 35A would have been more than adequate for the circuits it originally protected on the Sprite. If the fuse was designed to fail at 70A because it was new style rating, that's a lot of current!)
Graeme Williams

The original fuses were marked 17/35A - i.e. 17A continuous, 35A blow.

I would imagine, from what you have said, that you should be looking for a fuse marked 17A - or thereabouts.
Dave O'Neill 2

There is always great full coverage insurance and aluminum gum wrapper... easy enough

Prop
Prop and the Blackhole Midget

Dave: chasing this around on Google, it seems that the style of glass fuse marked 35A will carry 17A "for ever" but fail at 35A. Rimmer for example show this fuse (called "35Amp") and clearly state normal working current 17A, fail 35A. This is the old Lucas designation.
I have emailed Moss to clarify their fuse ratings as theirs are simply marked as "35A".
There doesn't seem to be a standard: some similar looking glass fuses are marked (say) 20A but this is the working capacity and won't fail below twice the marked value.
A MODERN fuse marked 35A will fail at 70A
Graeme Williams

It's a balance. For adequate protection, the fuse should melt, before the insulation of the cable being supplied by the fuse. For adequate supply, the fuse shouldn't melt, when supplying current continously to the device(s) at the end of the cable.

If you're increasing the current demand and the ability to supply it, then you'll have to increase the fuse and cable ratings.

If for example you are going to drive all night on main beam, and that requires 35A on that circuit alone, then a fuse that is only rated at 17A continous, won't be any good.

If you're worried, double the cable sizes, and increase the fuse ratings by a sufficient amount to supply the estimated maximum demand current, whilst still being rated to melt at a current flow below the maximum cable rating.

Or, fuse each device individually, and work out the demand from each fuse W/12v=A

Personally I wouldn't bother to add that many fuses.

Perhaps 2 were inconvenient, and knocked out too many other things if one failed. The later 4 box seems about right to me. So I'd just copy that and it's ratings.


Lawrence Slater

But that's not the point Lawrence! I am concerned about fuse marked ratings ie if a fuse is marked 30A is that the normal running current (with a blow rating of 60A) or is that the blow rating (and running current half that value?
The standard changed in the 80's and it's often not clear which standard has been used both on the fuse itself, or on the wiring diagram.
So take two similar looking 30mm fuses labelled "20A"
One may be designed to blow at 20A (old standard) and the other blow at 40A (new standard).
My understanding at the moment is that if you replace the standard glass 35A fuses box in the sprite with a blade fuse at 35A, buy a fire extinguisher!
Graeme Williams

Then ask someone like these people.
http://www.littelfuse.com/products/fuses/automotive-aftermarket-products/glass-fuses.aspx

There must be similar in the UK.
Lawrence Slater

Ask here. I assume from a quick read, that you buy the rate you want it to melt at.

http://www.ebay.co.uk/itm/Quick-Fast-Blow-Glass-Fuse-Fuses-2A-5A-10A-15A-25A-35A-50A-Various-Amps-/281602178536

David Pilgrim
69, Higher Road
Urmston
Manchester
Greater Manchester
M41 9AP
United Kingdom

Phone:01612838331
Fax:01617467670
Email:ph@qxcomponents.co.uk

Lawrence Slater

"I am about to rewire my Frog and will add a few extra fuses where appropriate"
8 fuse holders are readily obtainable which should allow you the flexibility needed. Then borrow an ammeter from someone and measure the current through each fuse (with all loads running). From here you can then get the correct fuses.
FWIW Cheers Rod
R W Bowers

This thread was discussed between 24/08/2015 and 26/08/2015

MG Midget and Sprite Technical index

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