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LUCAS FUSE RATING

 
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Bally
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Joined: 19 May 2011
Posts: 219
Location: ACT

PostPosted: Mon Jul 20, 2015 7:52 pm    Post subject: LUCAS FUSE RATING Reply with quote

G’day all
Because the lighting circuits on a MM are not fused, along with the fuel pump and ignition coil, I decided to fit an inline blade fuse holder in the wire no. 36 from A1 on the control box to A on the lighting switch. The fuse is close to the light switch and accessed by removing the centre grill panel. The circuits fed from this fuse are those from the lighting switch being the main beam at 10 amps., parking lights 0.4 amps, tail lights 1 amp, no. plate light 1.0 amp, head and side warning lamps 0.1 amp, instrument lights 0.75 amps, This gives a total of 13.25 amps. Also from A on the lighting switch a feed goes to the ignition switch which feeds fuse A3 – A4. This feeds the coil 2.0 amps and the fuel pump 2.0 amps from the live side of the fuse being an extra 4.0 amps. From the load side of the fuse is fed the trafficator at 2.5 amps, stop lamps at 3.0 amps, DR2 wiper at 2.0 amps and fuel gauge say 0.5 amps giving an extra 8.0 amps.. This would give a total loading on the new fuse of 25.25. In view of the fact that not all circuits were likely to be energised at the same time I decided to use a 25 amp fuse in this new fuse holder. Recently a problem occurred with the left trafficator and resulted in the new 25 amp fuse blowing. The fault with the trafficator was finally traced and fixed, however if I had not fitted the 25 amp fuse it would have then relied on the 35 amp fuse in the A3 - A4 fuse holder to supposedly protect the wiring by blowing. The wiring to the trafficator is 14/.012” (14 strands of wire .012” dia.). This is equivalent to 1.022 mm. squared which Lucas rate at 7.0 amps and Narva or SCA rate at 10 amps. It is possible for a 35 amp 3 AG fast blow fuse to not blow for up to 20 seconds when carrying 200% of it’s rated current. Imagine the 10 amp trafficator cable carrying 70 amps for 20 seconds! No bloody way! The circuits connected to the load side of the A3 –A4 fuse are trafficators 2.5 amps, stop lamps 3.0 amps, DR2 wiper motor 2.0 amps and fuel gauge estimated 0.5 amps, giving a total load on the A3 – A4 fuse of 8.0 amps. These circuits would almost certainly be fed by 14/.012 cables (10.0 amps) and are also supposedly protected by a 35 amp fuse which has the ability to carry 70 amps for up to 20 seconds! Because I have added indicator lights (blinkers) to my trafficators, these are 21/5 watts front and rear with the filaments wired in parallel, it results in an extra 4.33 amps drawn from the A3 – A4 fuse resulting total of 12.83 amps. The best I could come up with was to replace the 35 amp fuse in the A3 – A4 holder with 15 amp fuses. This is still not perfect but will provide the 12.83 amps required and afford infinitely better protection for 10 amp cables than the 35 amp fuse recommended by Lucas. I have attached 2 links, one from Lucas showing cable sizes and ratings and another from Littlelfuse showing the Electrical Characteristics for 312/318 Series Fast Acting Fuses.
Hope this is of value
Cheers Bob
https://fromtheframeup.com/uploads/Lucas06_Vehicle_Wiring_Circuits.pdf
http://www.littelfuse.com/data/en/data_sheets/312_318.pdf


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Bally
At rest


Joined: 19 May 2011
Posts: 219
Location: ACT

PostPosted: Tue Jul 21, 2015 3:22 pm    Post subject: LUCAS FUSE RATING Reply with quote

G'day all
G’day all
Further to my last post I would like to point out a potential hotspot under the dash, being the poor Lucas design for attaching the wires to the light switch. I have seen switch terminal posts discoloured and wire insulation charred as a result of being heated by poor high resistance terminations. I have screwed 6 mm. male spade connectors to my switch and crimped 6 mm. female connectors to the wires. This makes for much better connections which can easily be removed and replaced and virtually eliminates the potential fire hazard at this location.
Cheers Bob
http://i1373.photobucket.com/albums/ag395/bally/LIGHT%20SWITCH/P7210625_zpsy648xcpd.jpg[/img

http://i1373.photobucket.com/albums/ag395/bally/LIGHT%20SWITCH/P7210624_zps2yexkpo7.jpg


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Scouse
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Joined: 15 Sep 2003
Posts: 5266
Location: Wollondilly, NSW

PostPosted: Thu Jul 23, 2015 12:13 pm    Post subject: Reply with quote

When fusing the headlight circuit, I split LH & RH into 2 fused circuits. That way if one circuit shorts & blows a fuse, you still have the other headlight operational.
I do the same on high beam so I use 4 fuses all up just for the headlights.

If retaining a single circuit for the headlights, a self resetting circuit breaker might be better than a fuse.


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Bally
At rest


Joined: 19 May 2011
Posts: 219
Location: ACT

PostPosted: Thu Jul 23, 2015 1:03 pm    Post subject: LUCAS FUSE RATING Reply with quote

G'day all
Thanks Scott - I am looking at alternative ways now.
Cheers Bob


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