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Fusing the brown wires

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ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
Quote: The size of the fuse didn’t matter because it was a 625 CCA battery going direct to ground.

Exactly - and that is why an 80 amp fuse on the brown wire is probably safe. During a dead short event, the current is very high (well over 80 amps) and the fuse will blow within a second (and probably a fraction of a second). Although the wire is carrying much more current than it could safely sustain, it can easily carry 200 amps for a fraction of a second without getting hot enough to cause damage.

60 amp is probably still safer on the MGB if it is heavy enough to withstand the spikes when devices first power on. And, the brown wire could withstand a constant 60 amp current without melting. (In theory, it could withstand a constant 80 amp load, but I'm pretty sure it would be somewhat warm!)



Terry Ingoldsby
terry.ingoldsby@DCExperts.com



Edited 1 time(s). Last edit at 2018-02-13 06:22 PM by ingoldsb.

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rjmc Avatar
rjmc Robert McCoy
Somersworth, NH, USA   USA
1965 MG MGB
Obviously the most vulnerable circuits in our cars are the unfused lighting and ignition circuits. A dead short is going to destroy something on an unfused circuit. On my car each of these circuits fried wires when something shorted. Luckily the smaller #18 wires acted as fusible links protecting the bigger feeds. If a #14 or #12 wire was directly shorted more damage would have happened.

But the factory felt those circuits were critical and should not be fused to avoid an incapacitating nuisance fuse blow. Many still advocate no fuses for these circuits. Advance Auto Wire who make good harnesses advocate not fusing the headlight circuit. Many others are fusing the headlight circuits. I don't hear much about fusing the ignition circuit but it is also vulnerable to underdash mayhem.



Bob McCoy
1965 Pull Handle

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
Quote: But the factory felt those circuits were critical and should not be fused to avoid an incapacitating nuisance fuse blow. Many still advocate no fuses for these circuits.

The compromise is self resetting circuit breakers. They trip out on a short, but reset a few seconds later. If the fault is intermittent you are still in business.

I had a 1968 Ford that had these breakers from the factory. When I switched to halogen headlamps, it did trip the breaker after about an hour of night driving. It was the longest 3 seconds in my life waiting for the breaker to reset!



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Rick Fawthrop Avatar
Rick Fawthrop Gold Member Richard Fawthrop
Seattle, WA, USA   USA
Robert let me tell you what I did with my 1974 B.
I removed the grille and physically inspected the wiring and serviced the bullit connectors.
The grommets that protect the headlite wiring on the front inner panels was gone.
So I installed some plastic spiral wrap that I had left over from a 750 volt box.
I found a rub on the top wires coming out of the fuse box. So I fixed that.
I found more rubs under the radio console. And the insulation on the back of the wires from the headlight switch was burned off.

So the point I am trying to make here is that if you are not going to fuse the brown wire inspect the wire harness.
Replace the bullit connectors that cause high resistance creating heat and protect the wires from rubbing on sharp edges.

I think that the overdrive wiring and dash light rheostat should be fused as those are common wiring shorts to ground issues.

rjmc Avatar
rjmc Robert McCoy
Somersworth, NH, USA   USA
1965 MG MGB
Richard, thanks for the sound advice. I did an inspection and found the grommets thru the fenders to the lamps need replacement plus there is a lot of wrapping missing.

If I am not going to fuse the ignition and headlamp circuits I need to make sure they are in great shape and make sure there are no bare terminals.



Bob McCoy
1965 Pull Handle

Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
In reply to # 3680930 by rjmc . Advance Auto Wire who make good harnesses advocate not fusing the headlight circuit.

That's not good design. The current standard automotive practice is to fuse headlamp filaments and side lights separately so you avoid the common mode failure. Depending on the cost of the car that's either fuses or smart FETS (on my MG I've used fuses). The use of self resetting circuit breaker can lead to problems due to the load pulsing on and off, building up heat in the harness bundle which can cause more damage.

On my car I've fused the overdrive, the fuel pump and the fascia lights - all are easy to do with no wire cutting. I've also fused the radio and the electric windows. I plan to install a mega fuse into battery negative terminal and also onto the cooling fan.

Any fuse needs to be added at the correct point in the harness to provide adequate protection. I've got a marked up circuit diagram of my car showing the protected and unprotected circuits. Some circuits are difficult to protect without major harness changes, others are quite simple. On thing to be factored in is the probability of failure. Some unprotected circuits could have devastating effects if they shorted out however the cables are part of a bundle and are well clipped and protected - thus relatively low risk. Others e.g. OD switch on gear lever are very vulnerable to chaffing etc and need to be protected quite urgently. It's a matter of keeping it in proportion.

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
Quote: The use of self resetting circuit breaker can lead to problems due to the load pulsing on and off, building up heat in the harness bundle which can cause more damage.

I'm very doubtful about this. The breakers cycle on and off about every three seconds - so the harness is facing roughly a 50% duty cycle. And, obviously if they start cycling there is a major problem that requires swift action. The only purpose of the self resetting breaker on the headlamp circuit is to give you enough light to get the car safely stopped so the problem can be resolved. And what are the alternatives? Blow a fuse (permanently) and crash the car? Leave the circuit engaged until something catches fire (which turns off the lights and crashes the car)?

Whether you use simple fuses or power FETs, you can't get away from the fundamental choices:
1) Don't interrupt the power on a fault
2) Interrupt the power permanently
3) Interrupt the power intermittently

I would argue that, for the headlamp circuit, #3 is the best option.

Perhaps the best compromise is to separate the left and right headlamps onto separate circuits. Then, if one circuit fries, the other one remains functional.

But this discussion started out talking about the car's main power bus (brown circuit). And for that circuit the intent is to prevent a fire if some unfused wire shorts to ground. A short on the main power bus is going to stop the car anyway - it will kill the ignition (and the headlamps) so the only thing left is to prevent the car from catching fire.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
The best option is to fuse the filaments individually. When a fuse blows (or a smart device shuts an output down) you loose 1 filament, not the whole lighting set. This is the design required to meet current vehicle type approvals.
Designs using smart FETS often substitute filaments so if a dipped beam fails they’ll substitute a fog light or a main beam.
Current practice is to use leds instead of filaments. If one led in an array fails the array will still work though this is uncommon, the switch over being handled in the power converter.

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
I must say I wasn't aware that the industry is using MOSFETs as fuses - but it makes perfect sense. For those who don't follow electronics, modern FETs (Field Effect Transistors) make amazing switches. They can to from almost infinite resistance to a small fraction of an ohm in an incredibly short period of time - I would think a few tens of microseconds. Much, much faster than the fastest thermal fuse. Plus, they are easy to turn on and off with a computer signal.

And I agree completely - one fuse/FET per filament is the gold standard for fusing the headlamps. But, we were talking about a minor tweak to a stock wiring harness, not a complete re-engineering of the original design.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
In reply to # 3681329 by ingoldsb I must say I wasn't aware that the industry is using MOSFETs as fuses - but it makes perfect sense. For those who don't follow electronics, modern FETs (Field Effect Transistors) make amazing switches. They can to from almost infinite resistance to a small fraction of an ohm in an incredibly short period of time - I would think a few tens of microseconds. Much, much faster than the fastest thermal fuse. Plus, they are easy to turn on and off with a computer signal.

We've been using them on our high end vehicles for nearly 25 years (like many premium European OEMS) and now use them on all our cars. Smart FETS have very fast switching speeds and inbuilt diagnostics so they can detect open/short circuit, excessive current etc, protect themselves and report it back to whats controlling them.

On my B I've fitted relays to control the lights with a high current fuse to protect the relays and then individual fuses for each lamp filament. 5 fuses in all.

davcar Avatar
davcar David Carter
HERVEY BAY, Queensland, Australia   AUS
1969 MG MGB "Peggy-Sue"
1998 MG MGF "Lucy"
In reply to # 3681475 by Chalky
In reply to # 3681329 by ingoldsb I must say I wasn't aware that the industry is using MOSFETs as fuses - but it makes perfect sense. For those who don't follow electronics, modern FETs (Field Effect Transistors) make amazing switches. They can to from almost infinite resistance to a small fraction of an ohm in an incredibly short period of time - I would think a few tens of microseconds. Much, much faster than the fastest thermal fuse. Plus, they are easy to turn on and off with a computer signal.

We've been using them on our high end vehicles for nearly 25 years (like many premium European OEMS) and now use them on all our cars. Smart FETS have very fast switching speeds and inbuilt diagnostics so they can detect open/short circuit, excessive current etc, protect themselves and report it back to whats controlling them.

On my B I've fitted relays to control the lights with a high current fuse to protect the relays and then individual fuses for each lamp filament. 5 fuses in all.

Any chance of some photos showing your set up David would be appreciated.smiling smiley



The older I get, the more idiots I meet...!!!

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
And, could you provide some sample part numbers and sources for these devices? What "high end" manufacturer are you referring to?



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
Google is my friend. I found
https://www.edn.com/electronics-products/electronic-product-reviews/other/4426477/A-new-level-of-circuit-protection--The-e-fuse

Would you look at the specs on http://www.ti.com/product/tps25924/description ? 28 mOhms resistance. You'd have a hard time finding a wire with that low of resistance! smiling smiley



Terry Ingoldsby
terry.ingoldsby@DCExperts.com



Edited 1 time(s). Last edit at 2018-02-15 10:22 AM by ingoldsb.

riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
Maybe one should run the Brown wires outside the main harness? Just in case?

The Brown wires when fused become Purple wires! winking smiley

B



Check your ego Amigo!

Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
In reply to # 3681942 by ingoldsb What "high end" manufacturer are you referring to?

They’ve been used on Range Rover since 1994 and are used on other Land Rovers and Jaguars. BMW use them in the LSM light controller that’s been in use since the early 1990s. Rover 75 used the module as do Rolls Royce.Mercedes and VW/ Audi also use them.
The move to xenon and led lights has dropped their popularity since they use different control methods.

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