Earth strap current

[tonyishuk]

Out of idle curiosity , I wondered if anyone had made a chassis to engine connection and used a fuse to see if extra current is being drawn ?

 

I was thinking of a three amp, and if it pops, it's time to replace the original add an extra strap.

 

(Must have time on my hands *-) )

 

Rgds

[andytw]

I think I understand what you are saying.

 

The earth strap is big and fat because it carries a lot of current. It connects the engine to the chassis so any current used by or generated by the engine will pass through the earth strap. When the engine is started the starter motor will draw several hundreds of amps. Once running the alternator will generate tens of amps of current to charge the battery and power the lights etc.

 

If you put a fuse in series with the earth strap then it will blow. You will then no longer have an earth strap. When you then try and start the engine any current will take whatever path might be available. This could, for example, be the speedo cable which will then melt.

 

Without a good well bonded earth strap ANYTHING is possible.

 

I just re-read your post, maybe you are saying to add another cable with a fuse in it to see if the existing bonding is adequate or not. If this is the case then another strap of equal dimensions and fitted the same as the original would allow half of the current previously mentioned to pass through it so any fuse would still blow straight away.

[gordonaldson]

A little confused. I'm reading here that the earth strap is being used to supply power from the Alternator and Battery to starter and other services. I was always led to believe that the red wire {supply} carried the positive power and the earth wires, that one being one, between engine and body, was the return. Sorry if I'm wrong it's maybe just a dream I'm in...

[andytw]

You are right but it's the same thing. Suppose the starter is being cranked. Current will flow from the battery positive red lead to the starter motor, through the starter windings causing the starter to spin. The same current then exits the starter motor where the output is connected to the body of the starter motor. This is the ground connection and is now the whole engine but the current has to get back to the battery negative so it will travel through the ground strap to the body of the vehicle and then from the body of the vehicle up the black battery cable, which is connected to the chassis and back to the battery negative.

 

Charging the battery from the alternator is just the other way around...from alternator to battery positive, through the battery, out of the battery negative to the chassis, ground strap to the engine and back to the alternator -ve which is where it is bolted to the engine.

 

 

[tonyishuk]

Thanks for your replies.

 

I was working on the assumption that the bonded braided earth strap could take say 750 amps when new.

 

A cranking engine would draw say 500 amps, so the strap is OK when new

 

I was contemplating that the earth strap could degrade to an extent that it now will only carry say 496 amps, so requires extra amps via a second fused connection.

 

The difference of 4 amps would blow the 3 amp fuse.

 

Of course there could be such a wack of current being pulled, it could melt everything, fuse included.

 

Please note all currents quoted are pure speculation !

 

Don't know if that makes what I meant to say any clearer ? ;-)

 

Rgds

[andytw]

I understand where you are coming from but it wouldn't work like that. If you put 2 equal straps in parallel and the load id 500 A then each would carry 250 A. If one is slightly degraded, i.e. has some resistance and you put another good one in parallel then the new one will pass more current e.g. 300 A with 200 A going down the slightly degraded strap.

 

What you are trying to say is that if you have a brand new 100% earth strap and it is passing 500A then if you add another one in parallel, then nothing would go down it. This would require the electric current to be intelligent and know which route it is 'supposed' to be taking.

[Alanb]

Perhaps what would be more informative is to connect a multimeter set to "peak hold", between the battery negative terminal and the engine block. If I saw more than say 0.5 volts when starting the engine, I would be looking to reduce it.

[gordonaldson]

From what I can find if you ask any electrician what the return voltage would be they all say O volts. If a starter takes 12v 500amps to turn it there is not the same coming out the return cable.

In a cartoon scenario there is 12 little Newtrons at a switch waiting to light up a lightbulb. These 12 newtrons are strong and charged with many amps. Now the switch is pressed and they all run to the light, they heat the wire and it glows brightly. The light has now drained their energy and only one newtron emerges and knackered and crawling down the return wire.

So the earth return is only a return wire and nothing would run/power/light-up with out it.

As too the size of the engine/body earth brad it has to stand up to much abuse so a small cable would probably if not look silly.

[andytw]

Current leaving the battery is the same returning to the battery.

 

https://www.ibiblio.org/kuphaldt/electricCircuits/DC/00030.png

[Alanb]

To help reduce the confusion that may be experienced by those who are not electrically minded, may I point out that the diagram posted above by andytw, shows the direction of electron flow, which is technically correct. However conventional current flow is in the opposite direction, that is from positive to negative. The reasons for this conventional direction are historical.

 

As regards conductor size it has to be of sufficient cross sectional area (csa) and hence of such a low resistance, so as not to unduly impede the passage of current, or to overheat due to the power lost within it.

 

This conductor heating or power loss is what as known as I squared R loss. For example if we use the starter current value of 500A, and a conductor resistance of 0.01 Ohm (one hundredth) we get, via an algebraic manipulation of Ohms Law:

 

500 x 500 x 0.01 = 2500W or 2.5kW

 

This is as much as some electric kettles consume. This power has a heating effect on the cable, which is in direct proportion to the time for which it is applied.

[andytw]

This has deviated somewhat from the OP.

[euroserv]

Very interesting though