Fuses - continuous and blow amp values

Technical advice Q&A

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inca
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#1 Fuses - continuous and blow amp values

Post by inca » Thu Jan 26, 2012 3:04 pm

Hi

I would like to get some spare glass tube fuses for my 1969 Series 2 E type though a little unclear on the difference between continuous and blow amp values. In the fold down switch panel there is a small silver panel detailing all the fuses in the fuse board and what parts of the car electrics they cover.

On this panel it states all the fuses are 35amp with one 50amp fuse (cigar lighter) However on the glass tube fuse itself it has a piece of paper enclosed in the glass stating -

17A continuous
35AMP blow

Apologies for the dumb question but what does this mean and at the end of the day what fuses should I be keeping as spare for the car. Is it just straight 35 amp and 50 amp fuses.

In my local car shop the fuses do not state a continuous and blow rating so I guess this is a v old fuse I have?

Regards and thanks in advance of any replies

Mark

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christopher storey
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#2

Post by christopher storey » Thu Jan 26, 2012 3:39 pm

The Lucas glass fuses were always described in the workshop manuals by their blow rating i.e. the maximum current that could be carried transiently without the fuse blowing. The continuous rating ( usually about 50 to 60% of the blow rating) was the load which the circuit protected by the fuse was designed to carry. hence your 35Amp Lucas fuse was designed for a circuit load of no more than 17 amps . The more modern fuses use the American rating system, which has always worked on the circuit load basis i.e. the continuous rating. Thus, if you change to a new fuse box and use the plastic or ceramic type fuses, in general terms you should replace 50 amp fuses with new 25 or 30 amp ones, 35 amp fuses with new 15 or 20 amp ones, etc. What it is vital not to do , is to replace an old glass 35 with a new style 35 - this may well result in wiring fires if a short circuit arises, and the same applies pro rata to the lower value fuses

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Topic author
inca
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#3

Post by inca » Thu Jan 26, 2012 4:11 pm

Many thanks Chris.

So just to be sure - using the old fuse box I have and replacing with old style glass tube fuses if the information panel in the fuse box states 35 AMP fuse I should actually fit a 15 or 20 amp fuse and where it states 50 AMP I should put in a 25 or 30 amp fuse?

One final point. I am looking to plug into the cigar lighter socket a multi socket to charge my phone and power some small speakers. The 12v multi socket states Max 10 amps across both sockets. What is the calculation here to know whether the speaker and phone are within this limit?

Apologies of the dumb questions but electrics is not my strong point - not that I have any!

Regards

Mark

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christopher storey
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#4

Post by christopher storey » Thu Jan 26, 2012 7:48 pm

Mark ; Fuses : no - if you are sticking with the old glass fuses, you should use ones with the same rating as those you are removing. So, if you take out a 35 amp, use a fresh glass 35 amp . If you look at new and old side by side, the thickness of the fuse wire should be the same .

As far as your plug into the cigar lighter is concerned, you can assess the current demand by using the formula Amps = Watts divided by Volts . So, a 10 amp load on the socket represents a consumption of 120 watts on a 12 volt supply . Unless your speakers and charger use more than 120 watts together ( unlikely in my view unless you are putting a large set of self powered speakers in ) you will not overload either the socket or the plug

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Heuer
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#5

Post by Heuer » Thu Jan 26, 2012 9:26 pm

One other point - that multi-socket you are thinking of using will be a real pain because every time you change gear (1st, 3rd or 5th if you have it) your knuckles will hit and displace it. Better to go with something like this: http://etypeuk.com/forum/viewtopic.php?t=168
David Jones
S1 OTS OSB; S1 FHC ODB

Add your E-Type to our World Map: viewtopic.php?f=1&t=1810

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inca
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#6

Post by inca » Fri Jan 27, 2012 8:32 am

Many thanks for your help, it's really appreciated.

I mentioned in my earlier post the 50amp fuse for the cigar lighter , well checking last night it is actually the horn! Most of the car parts shops don't appear to stock fuses as high as 50amps!

Regards

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PeterCrespin
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#7 Re: Fuses - continuous and blow amp values

Post by PeterCrespin » Tue Mar 12, 2019 12:11 pm

‘Rutuja’ simply rephrases the posts of others using improbably contorted ornamental language with zero original input. This is clearly some asinine troll game he enjoys.

Today it is Chris Storey’s post above from 26 Jan and yesterday it was Julian’s tach post from March 5th.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 97 XJ6L

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mgcjag
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#8 Re: Fuses - continuous and blow amp values

Post by mgcjag » Tue Mar 12, 2019 1:02 pm

Hi Peter.....well spotted...i had already noticed...although not the usual nonsense from a spammer....so will monitor for now.....All the best....Steve
Steve
1969 S2 2+2 & Building a C type replica

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