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MG Midget and Sprite General - earth strap 2

I note from the other thread the discussion re the original braided earth straps fitted to our cars. My engine is earthed by a long battery lead from the bellhousing bolt to the bolt on the bulkhead to which the negative battery lead is attached.
I would have thought this to be a much better arrangement than the braided strap to chassis affair as it provides a direct run back to the battery from the engine and its electrical ancilliaries. I presume using a battery type cable is good enough? Do braided straps supposedly carry more current? If my theory is right why didn't they come like this from the factory instead of having a metal strap dangling in the muck?
I guess building down to cost, but wait to stand corrected!
Matt1275Bucks

braided are cheaper

your arrangement sounds good

I think FRM suggested an even better method than yours, think lead from battery to bellhousing close to or on starter motor joint and a second (perhaps slighly thinner?) lead from battery to body
Nigel Atkins

Braided is flexible. If you had a solid copper core, it could be thinner, but it would work harden and snap. So cable, where it needs to flex regularly is multi core.

The engine is in motion constantly accelerating and deceleraring. When it starts it can twist violently, esp if the mounts are shagged. Hence you need very flexible cores, so they won't snap after a few months.

The Braided earth strap was a factory fit.

Mine is a 1966. Mine still has the ORIGINAL braided earth from chassis to Engine/g/box bellhousing, and the ORIGINAL braided earth from Bat to bulkhead in the engine bay.

Why try and improve what doesn't need improvement?

Lawrence Slater

the multi-core aren't thin, they are installed in a U-shape that allows for movement, I'd guess they'd last as long as the modern cheap braided types if not longer

if the multi-core snaps after a few months because of violent twist from the engine then those mounts probably need changing :)

most non-original battery to bulkhead earth leads are multi-core

I don't think MG always put the best quality items on the car when new, building down to a price has always been about
Nigel Atkins

Hi mate

Firstly the vehicle body is a great conductor and is probably better than any cable it replaces.

The only issue with the body is the quality of the connection between the battery and the engine.

I do not think braided cable is any better or any worse than multi stranded battery cable. However what is significantly better is a cable called "UNIPREN" which is lower resistance than normal multistrand or braided cable.

However a good connection to the body and a good connection to the engine/gearbox will be more than adequate for the job. Connecting 2 cables to the battery one to the body and one to the engine seems difficult to do. My battery clamp tends to use the full allocation of cable simply to connect to the body. If you used UNIPREN then perhaps 2 lots could be fixed into the battery clamp and run to the body and the engine respectively.

Solid copper cable should NOT be used for reasons stated above but multi strand cable is just as flexible as braid.
Bob Turbo Midget England

Bob,
I can't see any prices but it looks expensive, mind being an ex hi-fi nut I've paid lots for bits of wire before

my battery connectors are screw to hold leads type and have plenty of room for another lead
Nigel Atkins

mine's a '66 too Lawrence. And yep.. the original braided straps still as good as new...
David Cox

Actually Nigel, braided cable is more expensive than twisted multicore. With a braid the individual strands are tinner, and thus more flexible.

"if the multi-core snaps after a few months because of violent twist from the engine then those mounts probably need changing :)" --- If it was a flat braid, the engine mounts could be shot for years, and the braid would outlive the multi-core.

As Rob says, the cross sectional area (csa) of the chassis, is far greater than the CSA of an earth lead run down to the engine/gbox, an thus even though it's steel the resistance is less. So a very short length of very flixible flat braid is more than perfectly adequate to act as the earth path. It also gets the cable out of the way.

Why is Unipren a better conductor than an equivalent csa of braided? It's just tinned annealed copper. Tin has a resistivity greater than coppper.

Like for like csa, if the braided was untinned copper, I would imagine that braided would be the lower resistance, assuming the same purity of copper.

Lawrence Slater

Sorry Lawrence for not answering before

As you rightly say the flexibility of any cable is dependent upon strand diameter. Why do you believe braid has smaller diameter strands than multistranded cable?

Multi stranded cable comes in a variety of strand diameters so can be easily as flexible as braid.

UNIPREN is especially small diameter and is thus lower overall resistance.
Electron flow at an atomic level (or should that be molecular?) flows along the surface of a conductor. As you will understand the more strands a cable has the larger the overall surface area thus more electron flow is possible (Less resistive)
Added to this is the calculation that resistors in parallel significantly reduce the overall resistance. Therefore each strand in a multistrand cable may be thought of as a single resistor. Then each strand becomes a resistor in parallel with each other strand making the overall resistance of the cable far lower than each individual strand.

Have you ever wired lights to a battery in parallel and seen how bright they are compared to the same lights wired in series.?

Anyway those are the reasons why UNIPREN (or any other fine stranded cable) is less resistive and can therefore handle far bigger current flows than heavier gauge cable.

For info

The current carrying capacity of cable is determined by its resistance. This is because resistance is a heating element so as current flows the resistance acts like a heating element and so the cable warms up proportional to the resistance value. Obviously we can not allow cable to get too hot so current limits are applied to each specification of cable.

I hope that was straightforward if boring!! :)
Robert (Bob) Midget Turbo

Hi Rob,

Me sorry too. I should have emphasised FLAT braid instead of saying braid. I meant flat braid is more flexible.:)

I don't think the argument you advanced about the current flow being on the outside of the conductor is correct. Aren't you referring to "skin effect"? This applies only in AC or pulsed high frequencies, due to induction.

In DC this doesn't apply. So the greater the CSA the lower the resistance in every case.

I can't see how a smaller csa can ever equal a lower resistance, unless you cahnge the material, Ally to copper, to gold for example.

But yes I agree, resistors in // are lower in resistance value than resistors in series.

For resistors in //, R=1/(1/r1+1/r2+ ...1/rn)
For resistiors in series, R = r1+r2+ ...rn

But only in DC, in AC you have to consider impedance.

Lawrence Slater

Now you are talking

Not sure if you have done an electrical degree but when I was doing mine I almost gave up on fully understanding AC theory whilst I found Calculus "relatively" easy LOL.

I think we ought to leave it at that before everyone loses the will to live! LOL
Robert (Bob) Midget Turbo

I confess to a past background in electrical/electronics :)

It's been a while since I have used it though. Use it or lose it they say, and it doesn't spring to mind as easily as it used to that's for sure. lol.
Lawrence Slater

This thread was discussed between 06/12/2011 and 07/12/2011

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