# An ammeter wiring question.



## Jimdear2 (Oct 12, 2008)

Pb Cruiser said:


> Should the wires going to the ammeter( inside the car ) from the HV shunt be fused in any way? I'm finallizing my enclosure for underhood components and I'm trying to think of everything. If they need to be fused would it be for the ammeter/shunt protection or the occupant's. I realize we're talking millivolts here; am I over-thinking this?


IMHO the only reason to fuse it would be the unlikly event of a high voltage system short somehow applying voltage in such a manner that it could get into the vehicle. A one in a billion chance maybe? Even then, if you used say 22 gage wire for the shunt to meter circuit (which is all you need for this circuit) the wire itself would be enough fuse.

In my opinion no fuse is needed, 

Others feel free to disagree, I may be missing something.


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## Woodsmith (Jun 5, 2008)

Pb Cruiser said:


> Should the wires going to the ammeter( inside the car ) from the HV shunt be fused in any way? I'm finallizing my enclosure for underhood components and I'm trying to think of everything. If they need to be fused would it be for the ammeter/shunt protection or the occupant's. I realize we're talking millivolts here; am I over-thinking this?


Actually the voltage would be pack voltage, the current would be milliamps across the ammeter with the bulk of the current going through the shunt.

Now if your pack was fully floating and there was a fault, say, one of the ammeter cables shorted to the metal of the dash then it would bring some pack voltage to the dash.
If it were the pack side of the ammeter then you would have pack voltage and as much pack current as the cable will carry but it has nowhere to go so nothing much will happen. If it was the load side of the ammeter then you will get pack voltage and milliamps to a few amps also with nowhere to go. You may get a little zap or you may get a big zap depending on what else you are touching at the time but there will be little warning.

Things happen when you have a grounded pack.

In the first case of a pack side fault then the ammeter cable will melt due to carrying full short circuit current.
In the second case of load side fault you will melt the coil in the ammeter.

Floating or grounded pack I would be tempted to fuse it on the pack side of the ammeter. It only needs to be a milliamp fuse but it would save the cable melting and all the things it could heat up in the process.
The ammeter itself would act as a fuse if the load side cable shorted.

But I am only saying this as a buildings electrician where we fuse everything and frown and tut at automotive circuits.


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## Guest (Aug 13, 2009)

Use a good quality wire and use a fuse. If you let the wire be the fuse and you do have a short you could start a car fire. Fuse all your connections from your HV side and your accessory side. It also only takes a milli second short to kill so yes you need to fuse. It is better safe than sorry and it is no big deal to take the extra time to build a system that is fully fused. Might save you from a vehicle fire or death. 

All cars have fused systems. Always fuse.

Pete


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## evlowrider (Jul 23, 2009)

Woodsmith said:


> Actually the voltage would be pack voltage, the current would be milliamps across the ammeter with the bulk of the current going through the shunt.


Correct me if i'm wrong but the voltage across the shunt is in the order of milli-Volts (_not pack volts_) as the original poster mentioned, since essentially an Ammeter in this case is just a volt meter measuring a voltage drop across the shunt terminals of a known resistance. Hence his question regarding needing fuse for a cable carrying milli-volts. 

Pete.


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## Guest (Aug 13, 2009)

Always err on the safe side. Even at pack voltage and milli amps you can toast a wire hot enough to start a car fire. Don't let those milli amps fool you. 

Fuse every thing. Don't skimp. It is easy and cheap to replace a fuse but not your car. 

Fuse Fuse Fuse and fuse some more. 


Pete 

Just make sure the circuit is fused with at least one properly rated fuse. All circuits should be fused. There is not one circuit on your ICE that is not fused, why would you leave them out on your build. There is a reason for fusing your system.


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## Coley (Jul 26, 2007)

Across the shunt, yes. But each wire is carrying pack voltage to the dash.

Test this with your voltmeter....


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## evlowrider (Jul 23, 2009)

Coley said:


> Across the shunt, yes. But each wire is carrying pack voltage to the dash.
> 
> Test this with your voltmeter....


I think I see what your saying now... that wire in relation to ground on the car is essentially pack voltage right? (excuse my noobishness here).

On the subject of fuses - does it matter where the fuse is located? i.e. would it be better to locate a fuse near the shunt or near the ammeter.

Also in general, are people putting fuses in centralised locations (i.e. routing such things to the existing fuse box or additional fuse box). This question really only applies to low voltage system of course.


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## Guest (Aug 13, 2009)

I use separate fuses for the HV and utilize the stock fuse box for low voltage accessory stuff. No HV ever sees the stock fuse box. I will be building a standard one location fuse box for my HV but for now fuses are placed around where needed. 

Pete


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