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Old 09-12-2012, 09:11 PM
albo2 albo2 is offline
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Default Ideas on how to comply with Ground Fault detection

does anyone know how to comply with this.
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Old 09-13-2012, 02:12 AM
Plymouth60 Plymouth60 is offline
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Default Re: Ideas on how to comply with Ground Fault detection

I haven't been through it yet, but was considering a device like this:

http://www.bender.org/products/groun...d-systems.aspx

Applications: IR425

Single-phase AC/DC systems up to 300 V
Systems with variable frequency drives (VFDs)
Control circuits
Portable generators
General low-voltage, ungrounded circuits
Large industrial systems

Cheers
John
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Old 09-13-2012, 04:16 PM
CrunchTime CrunchTime is offline
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Default Re: Ideas on how to comply with Ground Fault detection

I haven't been though this yet either, but note that the regs say "where the components are suitable", and there's a strong case to be made that series DC/brushed motors are not suitable since there's always some leakage current. Check with your certifier before spending too much effort on it (and report back here please )
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Old 09-14-2012, 02:17 AM
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Default Re: Ideas on how to comply with Ground Fault detection

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Originally Posted by CrunchTime View Post
I haven't been though this yet either, but note that the regs say "where the components are suitable", and there's a strong case to be made that series DC/brushed motors are not suitable since there's always some leakage current. Check with your certifier before spending too much effort on it (and report back here please )
Yep I read that however my conversion is AC induction I don't think I'll be able to get away with that, I have found a kit on jaycar electrical that measures voltage and can be set to switch a relay, if I connect that to the 12v system and set it to run an alarm if it sees say 20 volts wouldn't that work?
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Old 09-14-2012, 03:42 PM
CrunchTime CrunchTime is offline
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Default Re: Ideas on how to comply with Ground Fault detection

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Originally Posted by albo2 View Post
Yep I read that however my conversion is AC induction I don't think I'll be able to get away with that, I have found a kit on jaycar electrical that measures voltage and can be set to switch a relay, if I connect that to the 12v system and set it to run an alarm if it sees say 20 volts wouldn't that work?
I don't think it's the 12V circuit you need to worry about - it's the high voltage circuit. Also, running an alarm isn't good enough - it has to actually break the circuit. If the battery is sending 200A output one side, and only getting 100A back on the other, the other 100A is going somewhere it shouldn't (i.e. through you) this is a bad thing that needs to be stopped, and soon

My understanding is that ground fault detection works by looking at the current (NOT voltage) going out from the power source i.e. battery, and compares it to the current returning - they of course should be equal. If there is a ground fault, it will cause the return current to be less than it should (since by definition some of the current is following another path), which then trips a relay to disconnect the power.

I understand the actual current difference that trips the relay is usually in the mA range, and triggers the relay in milliseconds, so the actual current going through the wrong path is very short duration.

But this is all just from researching it when I first saw the requirement, and I didn't get to a solution then, nor do I claim to be an expert...
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Last edited by CrunchTime; 09-14-2012 at 03:45 PM.
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Old 09-15-2012, 04:05 AM
albo2 albo2 is offline
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Default Re: Ideas on how to comply with Ground Fault detection

Quote:
Originally Posted by CrunchTime View Post
I don't think it's the 12V circuit you need to worry about - it's the high voltage circuit. Also, running an alarm isn't good enough - it has to actually break the circuit. If the battery is sending 200A output one side, and only getting 100A back on the other, the other 100A is going somewhere it shouldn't (i.e. through you) this is a bad thing that needs to be stopped, and soon

My understanding is that ground fault detection works by looking at the current (NOT voltage) going out from the power source i.e. battery, and compares it to the current returning - they of course should be equal. If there is a ground fault, it will cause the return current to be less than it should (since by definition some of the current is following another path), which then trips a relay to disconnect the power.

I understand the actual current difference that trips the relay is usually in the mA range, and triggers the relay in milliseconds, so the actual current going through the wrong path is very short duration.

But this is all just from researching it when I first saw the requirement, and I didn't get to a solution then, nor do I claim to be an expert...
I guess if we used amps it would have to be done with hall effect sensors as I don't think you can put a shunt in the positive leg.

Just read through the bender site above and they have a DC detector that measures resistance to ground on both the positive and negative legs that would be the way to go, hmm I wonder how much they are, only one way to find out.

Last edited by albo2; 09-15-2012 at 04:40 AM.
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Old 09-16-2012, 04:22 AM
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Default Re: Ideas on how to comply with Ground Fault detection

Could any of you smart people tell me what if any would be the effect of the main pack leaking to ground on the 12V system, wouldn't that see a negative voltage ie the ground having a higher potential than the positive terminal of the 12v battery
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Old 09-17-2012, 04:42 PM
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Default Re: Ideas on how to comply with Ground Fault detection

Quote:
Originally Posted by CrunchTime View Post
...My understanding is that ground fault detection works by looking at the current (NOT voltage) going out from the power source i.e. battery, and compares it to the current returning - they of course should be equal. If there is a ground fault, it will cause the return current to be less than it should (since by definition some of the current is following another path), which then trips a relay to disconnect the power.

I understand the actual current difference that trips the relay is usually in the mA range, and triggers the relay in milliseconds, so the actual current going through the wrong path is very short duration....
You are correct. The difficulty with complying with rules like this is that it is totally impractical to resolve milliamps of difference between the forward and return current out of, e.g., 1000A. A typical leakage current trip spec in the US is 5mA; that's 5 millionths of the output of a 1000A controller. Needless to say, a current measurement circuit with 0.0005% accuracy is going to be *expensive*...

So the way this is typically done in traction circuits is to periodically measure the resistance between the two supposedly isolated systems. Note that the act of making this measurement creates a ground fault in itself, so the usual approach is to use a relay to connect the measuring circuit to the two isolated systems just long enough to take a measurement, then default to being disconnected so that isolation is maintained.

It's a laudable goal to look for current leaks like this, but it's an incredibly thorny problem to solve.
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Old 09-17-2012, 08:56 PM
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Default Re: Ideas on how to comply with Ground Fault detection

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Originally Posted by albo2 View Post
Could any of you smart people tell me what if any would be the effect of the main pack leaking to ground on the 12V system, wouldn't that see a negative voltage ie the ground having a higher potential than the positive terminal of the 12v battery
Errr, no. Leakage from the traction pack to ground will have no effect on the 12 V system.

It won't have any effect on the high voltage system, either, unless there is a second fault. But the Encyclopedia Galactica notes the effect of even small currents on certain carbon based bipeds, if they come in contact with the traction pack when there is a leakage to ground elsewhere.

[ Cultural reference to the original Hitchhiker's Guide to the Galaxy TV series (well, the radio series was the real original, but I mean not the Hollywood movie.) ]

Last edited by Coulomb; 09-22-2012 at 06:30 AM.
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Old 09-20-2012, 03:08 PM
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Default Re: Ideas on how to comply with Ground Fault detection

Well if you are using a 3 phase AC motor, would a possible fix be to use 3 of those RCD/ELCBs as sold by various hardware and electrical stores for about $15 each. Plugged into a standard wall outlet the appliance, or extension lead, plugs into it's controlled output. I use mine for using an electric lawnmower outside, or for if I have an appliance in bits for repair, and want to power it up temporarily. Wire up normal type household 10 amp sockets and install 3 of these as required, running plugs to your motor. But does your motor draw more than 10 amps each across all 3 phases ? The household models available everywhere are limited to 10 amps each phase obviously, however a wholesale/trade supplier like Radcliffe might have a 3 phase model that could handle 32 amps per phase as a similar plug-in unit. Try asking around.
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