im never satisfied with my earth mounts, and always think they could be better.
do you think soldering them to the body is going too far.
any tips aside from cleaning (sanding) under them and clear coat will be appreciated, especially to the block.
i think it was posted before but what is a good ohm reading from your battery to the point in concern.
is this the best was to mearure a good earth.
good earth
Started by
TerrA LX
, Jun 19 2006 09:48 PM
46 replies to this topic
#1
TerrA LX
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Posted 19 June 2006 - 09:48 PM
#2
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Posted 19 June 2006 - 10:03 PM
A good earth should be as close to zero ohms as possible. A typical multimeter will only read to the nearest 0.1 ohm, so even if it reads zero the real resistance could be as high as 0.04ohm which is not a very good earth at all. Should be looking for only thousands of an ohm.
You can measure the resistance of a connection by looking at the voltage drop across it. You need to have a current flowing in the circuit.
Put your mulitmeter on the 0 - 2000mV scale and measure the voltage across the connection when you have a significant current. The higher the voltage drop, the higher the resistance...ideal = zero voltage drop. If you use something with a known current load, then you can calculate the res using v=ir.
In terms of making the connection, I dont think putting a layer of paint between the two surfaces is going to help. it would help to cover the bits youve scraped to bare metal that are exposed to the air rather than contacting each other to stop corrosion. Soldering can provide some protection against corrosion, but the force holding the connectors together is what you should concentrate on.
You can measure the resistance of a connection by looking at the voltage drop across it. You need to have a current flowing in the circuit.
Put your mulitmeter on the 0 - 2000mV scale and measure the voltage across the connection when you have a significant current. The higher the voltage drop, the higher the resistance...ideal = zero voltage drop. If you use something with a known current load, then you can calculate the res using v=ir.
In terms of making the connection, I dont think putting a layer of paint between the two surfaces is going to help. it would help to cover the bits youve scraped to bare metal that are exposed to the air rather than contacting each other to stop corrosion. Soldering can provide some protection against corrosion, but the force holding the connectors together is what you should concentrate on.
#3
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Posted 19 June 2006 - 10:30 PM
"Star" washers between earth terminal and body should be sufficient. Bites into material reguardless of paint.
Edited by rodomo, 19 June 2006 - 10:31 PM.
#4
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Posted 20 June 2006 - 12:29 AM
as ^ said star washers are best for electrical contact but a good split washer will do the same job"Star" washers between earth terminal and body should be sufficient. Bites into material reguardless of paint.
some people also run a cable from engine-chassis to help with the body ground
tom
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Posted 20 June 2006 - 07:56 AM
Star washers are good for those connections that must bite through paint, like earthing the lamps in your boot, but believe if you are trying to minimise contact resistance on a large current carrying wire, such as in your starter circuit, that the best contact will be made between two bared contacts of as large surface area as possible.
#6
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Posted 20 June 2006 - 09:43 PM
Just thought id point out a couple of things.
2000mV is only 2 volts, not high enough for 12v accessories on our cars, the only way to tell if you have a good connection is with an ohm meter that can read very low resistances - 0.001 ohms or a volt meter that can read very accurate voltages - 0.001 volts that can do that accuracy at 12V.
You COULD calculate the resistance difference if you know the accessory current rating and what its actually dragging, eg:- fan motor rated at 10A but draws 10.005A, but this method is inaccurate and you need to test the current draw very accurately because we are talking about very low resistances. The method is inaccurate because the equipment rating isnt accurate enough.
and these are all of ohms laws formulas fully transposed for everyones convenience, I also included all the power formulas...
V = IxR (This means current in amps x the resistance in ohms equals the voltage)
I = V/R (volts divided by the resistance yada yada, you get the picture)
R = V/I
P = VxI (This is power in Watts)
P = RxI2 (Thats says I squared)
P = V2/R
I = P/V
V = P/I
R = V2/P
R = P/I2
And theres these last two, dunno how to do a square root symbol on the computer so...
I = root P/R
V = root PxR
And the best connection is just as devil said - two bare metal surfaces with the largest contact area that is possible and clamped together as tight as possible, and use lugs on your cables that are crimped tightly.
If thats good for 1000A then its good enough for our cars!
Paint OVER THE TOP of the connection if you want to avoid corrosion.
Cheers Anthony.
2000mV is only 2 volts, not high enough for 12v accessories on our cars, the only way to tell if you have a good connection is with an ohm meter that can read very low resistances - 0.001 ohms or a volt meter that can read very accurate voltages - 0.001 volts that can do that accuracy at 12V.
You COULD calculate the resistance difference if you know the accessory current rating and what its actually dragging, eg:- fan motor rated at 10A but draws 10.005A, but this method is inaccurate and you need to test the current draw very accurately because we are talking about very low resistances. The method is inaccurate because the equipment rating isnt accurate enough.
and these are all of ohms laws formulas fully transposed for everyones convenience, I also included all the power formulas...
V = IxR (This means current in amps x the resistance in ohms equals the voltage)
I = V/R (volts divided by the resistance yada yada, you get the picture)
R = V/I
P = VxI (This is power in Watts)
P = RxI2 (Thats says I squared)
P = V2/R
I = P/V
V = P/I
R = V2/P
R = P/I2
And theres these last two, dunno how to do a square root symbol on the computer so...
I = root P/R
V = root PxR
And the best connection is just as devil said - two bare metal surfaces with the largest contact area that is possible and clamped together as tight as possible, and use lugs on your cables that are crimped tightly.
If thats good for 1000A then its good enough for our cars!
Paint OVER THE TOP of the connection if you want to avoid corrosion.
Cheers Anthony.
#7
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Posted 20 June 2006 - 11:44 PM
Torana 428hp:Just thought id point out a couple of things.
2000mV is only 2 volts, not high enough for 12v accessories on our cars, the only way to tell if you have a good connection is with an ohm meter that can read very low resistances� - 0.001 ohms or a volt meter that can read very accurate voltages - 0.001 volts that can do that accuracy at 12V.
The question posed was how to measure the resistance of a "good" earth.
The voltage drop across the contacts of an earth will be very small(if its more than 2V its a very poor earth in a 12V system), thats why i suggested using the 0-2V scale on a multimeter,giving 0.001V resolution, if you use the 0-20V scale it will read 0.00V even with a mediocre earth, a reading may be obtained on the 0-20V scale if you were using more than 100A as your test load..but still wouldnt exceed the 2v scale. Thes test is best done with a mild load like 10A(which would be about the load of turning on the headlamps), as it wont be subject to how the emf of the battery will continually drop during a larger current test. Voltmeters that offer 0.001V on the 0-20V scale would be very expensive or I dare say dont exist, whereas 0.001V resolution on the 2v scale is available for $10. Im not aware of any ohm meters that measure tiny resistances either, the limiting factor would be the quality of the contacts that are made with the probes, that is why measuring the voltage drop is the preferred method.
Edited by devilsadvocate, 20 June 2006 - 11:57 PM.
#8
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Posted 21 June 2006 - 12:07 AM
i think ive missed the point here, what ammount of voltage drop is acceptable.
ie at 12v - 0.02v?
ie at 12v - 0.02v?
#9
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Posted 21 June 2006 - 12:23 AM
The voltage drop that is acceptable depends on how much current is going through the connection. If there is no current flowing there should be zero voltage drop. If a 10A load is drawn(like headlamps), then if you get anything more than 0.002V then the earth is suspect. I would only get excited about doing something about it if you have this happening anywhere in the connections that go to the starter and battery return.
You may have a 0-200mV scale on your multimeter too, some do......some dont, a very good earth would show up as <0.2mV(0.0002V) at 10A.
You may have a 0-200mV scale on your multimeter too, some do......some dont, a very good earth would show up as <0.2mV(0.0002V) at 10A.
Edited by devilsadvocate, 21 June 2006 - 12:27 AM.
#10
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Posted 21 June 2006 - 12:37 AM
alright i think ive got it.
so the piont of test must have a load on, like head lights on.
so if i test this and have 12.42 at the batt and 12.40 at the light then this is not acceptable.
am i on the right track here.
so the piont of test must have a load on, like head lights on.
so if i test this and have 12.42 at the batt and 12.40 at the light then this is not acceptable.
am i on the right track here.
Edited by ALX76, 21 June 2006 - 12:42 AM.
#11
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Posted 21 June 2006 - 12:56 AM
That would be sensational if you only lost 0.02v through the whole connecting circuit. Id expect to see 0.4-5V difference in the voltage across the headlights compared to the batt voltage.
The test mentioned was testing a good earth/connection,just the voltage immediately across where a wire terminates and joins another conductor such as the body of the car. I also mentioned you would need to be using the 0-2V or 0-0.2V scale on your multimeter?
A whole circuit includes many connections + all the conductors(wires)
The test mentioned was testing a good earth/connection,just the voltage immediately across where a wire terminates and joins another conductor such as the body of the car. I also mentioned you would need to be using the 0-2V or 0-0.2V scale on your multimeter?
A whole circuit includes many connections + all the conductors(wires)
Edited by devilsadvocate, 21 June 2006 - 12:59 AM.
#12
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Posted 21 June 2006 - 01:07 AM
now i get it, thanx for that.
with the block earth wire is best to clean the surfaces install a star washer, tighten then paint over?
with the block earth wire is best to clean the surfaces install a star washer, tighten then paint over?
Edited by ALX76, 21 June 2006 - 01:14 AM.
#13
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Posted 21 June 2006 - 09:14 AM
As previously stated, best results with max area of contact-star washer wont achieve this.
#14
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Posted 21 June 2006 - 05:17 PM
Devil i cant say ive tried this, you may know something that i dont, but i dont think you can put a volt meter in series across a wire to measure voltage drop, they dont work like that, the only test you can do with the meter hooked up like the way your saying is a resistance test.
This is how i measure voltage drop in a circuit run at work...
I have a book - AS/NZS 3008, this book has values of millivolt drop per ampere metre for various conductors. This is the electrical cable selection standard
Vc = 1000xVd
----------
L x I
Vd = L x I x Vc
------------
1000
where:
Vc - the millivolt drop per ampere metre route length of a circuit, as shown in the tables for various conductors, in millivolts per ampere metre (mV/A.m)
Vd - actual voltage drop, in volts
L - route length of circuit in metres.
I - the current to be carried by the cable, in amperes.
2000mV scale on your multimeter is for testing voltages up to 2V
There are meters out the for testing very accurate stuff but they would be up around the $20,000 mark.
That is the only way to find out how good your connection is without buying a super meter. and when you tested that you had it on the 20V scale, not 2000mV scale.
This is how i measure voltage drop in a circuit run at work...
I have a book - AS/NZS 3008, this book has values of millivolt drop per ampere metre for various conductors. This is the electrical cable selection standard
Vc = 1000xVd
----------
L x I
Vd = L x I x Vc
------------
1000
where:
Vc - the millivolt drop per ampere metre route length of a circuit, as shown in the tables for various conductors, in millivolts per ampere metre (mV/A.m)
Vd - actual voltage drop, in volts
L - route length of circuit in metres.
I - the current to be carried by the cable, in amperes.
2000mV scale on your multimeter is for testing voltages up to 2V
There are meters out the for testing very accurate stuff but they would be up around the $20,000 mark.
alright i think ive got it.
so the piont of test must have a load on, like head lights on.
so if i test this and have 12.42 at the batt and 12.40 at the light then this is not acceptable.
am i on the right track here.
That is the only way to find out how good your connection is without buying a super meter. and when you tested that you had it on the 20V scale, not 2000mV scale.
Edited by Torana482HP, 21 June 2006 - 05:21 PM.
#15
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Posted 21 June 2006 - 05:51 PM
you cant measure volt drop by putting the meter in series with the connection, it must be placed in parallel with the connection (ie one ohmmeter probe on the conductor, the other on the chassis.
Torana 482
AS 3008 is the cable selection standard for mains wiring in Australia.
The formula in this standard are for AC, and not DC, and the mV/A.m are for standard solid and stranded 600v/1kv cables as specified ( whether they be TPS, neutral screened etc.
This formula will not apply to automotive cable (unless you are using a bit of TPS
cheers
dave
Torana 482
AS 3008 is the cable selection standard for mains wiring in Australia.
The formula in this standard are for AC, and not DC, and the mV/A.m are for standard solid and stranded 600v/1kv cables as specified ( whether they be TPS, neutral screened etc.
This formula will not apply to automotive cable (unless you are using a bit of TPS
cheers
dave
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Posted 21 June 2006 - 05:59 PM
i was just proving my point that you cant measure voltage drop with a meter.
The formula applies, the millivolt values obviously do not.
The formula applies, the millivolt values obviously do not.
Edited by Torana482HP, 21 June 2006 - 06:04 PM.
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Posted 21 June 2006 - 06:47 PM
Ive no idea what you are claiming here Torana482hp? Voltage drop can be measured with a meter.......did you mean to say something else.i was just proving my point that you cant measure voltage drop with a meter.
The formula applies, the millivolt values obviously do not.
I think Daveh has put you right but, loosely in electrical terms, putting something "across" something means putting the device/load/meter in parallel.
Placing a voltmeter in parallel with a connection measures the energy per coulomb(6e18 electrons) needed to cross that point. If no restriction there will be no energy change per electron as it passes the contact, ie zero voltage and one would conclude zero resistance. This works and doesnt rely on having great contact with the probes.
Not wishing to be patronising(I realise you are in the electrical trade), but perhaps if you consider how an analogue ohmmeter and voltmeter are just ammeters wired up differently it will make more sense:
The ohmmeter has an internal battery that is effectively in series with an ammeter, and a dumby resistor load in it designed to show full scale deflection (0 Ohms) when the probes are brought together. The ohmeter puts a voltage difference across the connection and measuring the resultant current flow and the scale of the ammeter is calibrated to give a resistance reading.
An analogue voltmeter consists of a ammeter and large resistor in series. Current will only flow through it if there is a voltage difference on the terminals.
Edited by devilsadvocate, 21 June 2006 - 06:49 PM.
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Posted 21 June 2006 - 07:34 PM
well im gonna keep my mouth shut until i try out what you first said, cause ive never tried it and it might work.
What dave meant was that you need to put the meter in parallel across a load and that will give you the voltage, (12V).
BUT what you are saying is to put it in series, ALTHOUGH, you say to put it across a connection, so the connection is your "load" and because there is hardly no difference in potential between point a and point b, the meter will read a very low voltage - Shit devil i think your right - thats your voltage drop across the connection!
well you learn something new every day, they never taught me that at school, had to figure it out meself. I dont think dave knew that was possible either. Although i better test out the theory before i make a bigger dick of myself.
What dave meant was that you need to put the meter in parallel across a load and that will give you the voltage, (12V).
BUT what you are saying is to put it in series, ALTHOUGH, you say to put it across a connection, so the connection is your "load" and because there is hardly no difference in potential between point a and point b, the meter will read a very low voltage - Shit devil i think your right - thats your voltage drop across the connection!
well you learn something new every day, they never taught me that at school, had to figure it out meself. I dont think dave knew that was possible either. Although i better test out the theory before i make a bigger dick of myself.
#19
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Posted 21 June 2006 - 07:49 PM
technically, a joint with any resistance (negative bonded to earth for example) is a load, and hence you can measure voltage drop across the joint when current is flowing in the circuit, by placing a meter in parallel with the joint.
If you were to put the mwter in series, you would have to remove the negative from the earth, and the only value that can be measure this way is current, which is not what you want
obviously the black wire coming from the left is the negative of the circuit, and the grey on the right is the chassis
the red and black are the probes on the multimeter
hope this explains things a bit better
cheers
dave
If you were to put the mwter in series, you would have to remove the negative from the earth, and the only value that can be measure this way is current, which is not what you want
obviously the black wire coming from the left is the negative of the circuit, and the grey on the right is the chassis
the red and black are the probes on the multimeter
hope this explains things a bit better
cheers
dave
#20
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Posted 21 June 2006 - 07:55 PM
yep, that will do it.
i never realised i could use my meter like that (for voltage drop that is), never thought about it properly like i now have.
(nice pic dave )
i never realised i could use my meter like that (for voltage drop that is), never thought about it properly like i now have.
(nice pic dave
)
Edited by Torana482HP, 21 June 2006 - 08:00 PM.
#21
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Posted 22 June 2006 - 12:12 AM
DA is correct, even seen or used a current shunt? they work in exactly the same way. these have a known resistance, and as we techies are aware voltage drop is proportional to current flow, so as the current increases the voltage drop across the shunt also increases, and thats why we use mV meters across current shunts to read current.i was just proving my point that you cant measure voltage drop with a meter.
The cable from the battery terminal to the point of connection to the body of the vehicle becomes the current shunt, with unknown resistance.
So if we have an approximate known current and a measured voltage drop across this cable you can get resistance, so in this case if you measure 0.5 volts, with an approximate load current of 10Amps (with only the lights on for example) you have a cable resistance of R=V/I, or 0.5/10 = 0.05ohms, which is unacceptably high.
#22
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Posted 22 June 2006 - 05:29 PM
Ill be putting this method to test on the weekend, check all me earth connections and starter motor connections.
Thanks for setting me strait Devil, Davelh and Hotrodder!
Thanks for setting me strait Devil, Davelh and Hotrodder!
#23
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Posted 22 June 2006 - 05:35 PM
i work in the high voltage distribution game, and do a lot ov voltage drop testing
we do things a bit different, and inject 100 amps across a joint, then use a microohmeter to measure the resistance of the joint, and work out the volt drop from that (our joints must have no more than 2mV across each one, it can add up with the higher voltages)
glad i / we could help
cheers
dave
we do things a bit different, and inject 100 amps across a joint, then use a microohmeter to measure the resistance of the joint, and work out the volt drop from that (our joints must have no more than 2mV across each one, it can add up with the higher voltages)
glad i / we could help
cheers
dave
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Posted 22 June 2006 - 06:47 PM
Cool, HV would be an interesting side to learn about in the trade i work at a factory doing maintenance/fault finding and new installation work, but this is only my fifth year, got one more test to do then im qualified, me boss always says that im switched on more than some other people, but im still learning new things 
#25
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Posted 22 June 2006 - 08:39 PM
on thing to remember, you learn something new every day, and never be afraid to ask questions, otherwise you wont get anywhere in life.
HV is a good area to work in, but it is more fitting/mechanical than electrical (i am an electrician though), and takes a bit mor to learn after your basic wiring and testing. I work for SA's equivalent of CitiPower/Powercorp on votages from 11kv right through to 275kv, and get to travel all over the place.
A lot of our work at the moment is fault finding and repair due to high summer loads a lot of our plant has been over stressed and is crying out for some help.
If youve got any Q's i would be more than willing to help you.
cheers
dave
HV is a good area to work in, but it is more fitting/mechanical than electrical (i am an electrician though), and takes a bit mor to learn after your basic wiring and testing. I work for SA's equivalent of CitiPower/Powercorp on votages from 11kv right through to 275kv, and get to travel all over the place.
A lot of our work at the moment is fault finding and repair due to high summer loads a lot of our plant has been over stressed and is crying out for some help.
If youve got any Q's i would be more than willing to help you.
cheers
dave
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