MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - glass fuse

glass fuse - glass fuses

absolutely no one has asked for or about this - yet here it is

(sorry about the not so good photo and my lack of measuring callipers)

photo below-

the top and bottom fuses are the modern equivalent 35 amp – they are 30mm x 6mm (approx.)

the middle fuses are two versions of the original type of 17 amp continuous/35 amp blow – these are 28.5mm x 6mm (approx.)



Nigel Atkins

...and just for the record, the modern 35A fuses are not equivalent to the 17A ones in the middle



N


Norm Kerr

interesting Norm as those were sold to me as the equivalents (I shouldn't have called them modern more like slightly more modern)

on the end caps of the 'slightly more' modern fuses in the previous photo are marked 35A

below is an earlier photo that was even more out of focus, I used the spare fuses taken from my car, these were bought from MGOC Spares as - GFS3035, 35 amp pack (x5) and GFS3035S for singles fuses

they are the same size, about 30x6, but the metal fuse strip width is not quite as wide perhaps this is (more?) accurate


Nigel Atkins

the "modern" 35A fuse is a 35A fuse (the US, "BUSS" fuses are rated only at "slow blow" current")

the older fuse with the paper inside, is a 17A fuse, which is its slow blow current. It also has a "35A" number on it, but that is its fast blow current, and is extremely confusing that it is shown, because it has no relation to a BUSS fuse size.

Any auto shop who sold you that 35A BUSS fuse and said it was equivalent was very, very wrong to do so (note how large its metal fuse is inside, so much bigger than the skinny wire in the other ones).

I wonder how many electrical fires in British cars were caused by people using fuses twice the rating of what was meant to be in there, because of this nomenclature difference?



Norm
Norm Kerr

Hi Norm,
thanks for your help

I can see that in the first photo those 'modern' fuses might be wrong they do look a thick link, probably why I put them in the fuse tin and bought a pack from MGOC Spares as in the second photo, so they should be right

I might be getting confused, very easily done, are you saying they're wrong too or have I misunderstood

the (US) BUSS system is different to our (UK) BS
Lucas Fuse Rating (British) 35 - BUSS AGC Equivalent (American) 25

Lucas part number 188218 – 17 Amp continuous/35 Amp blow

http://www.ebay.com/itm/OE-Commercial-Ignition-SP-5020-Lucas-188218-17Amp-35Amp-10-Pack-English-Fuses-MG-/380668338364
Nigel Atkins

Hi Nigel.

I must admit, I've never given much thought to the time ratings of fuses in cars. Just assumed that if you buy an auto fuse (generally dc), it's properly rated. But anyway. As you know, fuses blow( - or more accurately melt -) when too much current flows in them. They are there to protect devices and wiring(and people) from over current. They are rated in such a way as to present a low resistance to current below a determined level. Current flowing in any conductor, including fuses, causes heating. Too much heat, and the fuse or conductor gets hot enough to melt.

With a fuse, you want it to be able to carry the rated current demand of the circuit, ideally without getting hot, and certainly not hot enough to melt. But if too much current flows, either caused by a short circuit to ground, or by connecting too much demand, then you want the fuse to melt "quickly" to protect the wiring.

Hence fuses are rated according to how much current they can carry continously, and at the current at which they will melt within a set time. The time taken to melt it is important, because if the short or over demand is allowed to continue for too long, it all goes up in flames. So at the maximum rating you want a fuse to blow in less than a second.

Here's a quick guide to fuse selection. One of many on the web.
http://www.optifuse.com/PDFs/FuseSelectionGuide_RevA.pdf

So your 17/35 fuses should carry 17 indefinitely and I assume, melt "instantly" at 35. But it would be interesting to see the spec of those used in our age cars to see what "instantly" is defined as. Here's a table for a modern blade type fuse.

Specification Table
Rating (%) Blow Time
110% 4 Hours, Minimum
135% 0.75 to 1,800 Seconds
200% 0.15 to 5 Seconds
350% 0.08 Seconds, Minimum - 0.25 Seconds, Maximum

Lawrence Slater

I almost put I understand (pots and pans terminology now, I don't know the technical details just the general idea) fuses melt from overheating over a period of time and if they 'instantly blow' then there is/was something dramatically wrong and I know fuses can be slow(?), medium(?) and fast 'blow'

I understand that the 17 is for carrying the electric 'continuously' day in day out (corresponds to the 17 amp wiring(?)) but the 35 must be for a reason, is 34.99999 amps the maximum safe load/electric the wires can carry for an extremely brief time or a number in a rating system

I wondered if Norm though my blue paper 35A labelled fuses were incorrect for use in the car and that he might have forgot the different UK rating system (and perhaps that I'm in the UK so buying (hopefully) UK spec items)

I might have forgot but I can't ever remember a fuse blowing in any of my cars - I did have a dodgy modern blade fuse in one of my more modern cars though and I've seen/had a glass fuse that looked fine to the eye but didn't work
Nigel Atkins

I converted my car to blade fuses in a modern holder.

I use the lowest possible fuse that doesn't blow.
Mostly 5 or 10 A.

headlights are fused separately, through a relay.
Hugh Alison

This thread was discussed between 06/10/2013 and 08/10/2013

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.