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  1. #1
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    Default How do earth leakage (RCD) devices work with generators mounted in a truck

    Hi all,

    I have always wondered how the following system works;

    At work we have a generator installed in the truck. It's not installed permanently or electrically, just has a dedicated spot and held in place with an appropriate tie down. In use, we can either use it in the truck or remove it and place it on the ground. Either way, it is electrically isolated from the truck and the ground. We run waterproof extension leads and lighting towers for scene lighting (although these are being phased out with ELV LED lighting)

    The extension leads run to a distribution board which has built-in RCD protection. My question is this;

    Since the generator has no specific grounding wire from the chasis to the ground, even if there was an earth leakage from any part of the circuit to the ground, no current would flow anyway and no one should ever be electrocuted since there is no current path from the ground back to the generator. Why do we hav an RCD installed AND how can it work?

    Sure, we could add an earth stake from the generator to the ground but we dont and that would just make the system less safe and then we would really need the RCD?

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  2. #2
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    Not being an electrical person, I'm not sure whether I am correct in this or not but here goes -

    In 'normal' use (everything is working correctly), the earth does nothing. Current circulates from active to neutral and so on. The theory behind the earth is that if there is a fault, the current can flow to 'earth' rather than through a person touching the faulty item. A person's resistance is higher. Without that earth wire, all the current would flow through the person. With the earth (it having lower resistance), current flows mainly through the path of least resistance.

    RCD's rely on the current in being the same as the current out - that is, current through the active should be the same as current through the neutral. Normally if there is a fault to earth, the RCD 'sees' the in-balance and trips.

    On a device without a (low resistance) ground connection, an RCD should still be operable as the earth/ ground has nothing to do with what the RCD monitors. All it needs to operate is for current to be leaking away from it's correct path. While there may not be a proper ground connection, I would not wish to bet my life that the current could not find an escape path to ground it if wanted one. I would suggest that on un-grounded equipment, a functioning RCD is more important as in the absence of the earth connection, the only path for stray current is through a person, and while a potential circuit may not be there all the time, sooner or later one will be established.

    Michael

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    [EDIT - beaten to the punch line by MG]

    RCDs measure/monitor the current in and out of a circuit/appliance/machine. In its simplest form, they measure the current going back and forth in the active (or positive) and check if it's the same as that going back and forth in the neutral (or negative). If they're not he same there is a leak somewhere - maybe through a human to earth causing a heart seizure, so open the breaker. No separate earthing is needed.

  4. #4
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    Yes, I see your point.

    Thanks.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  5. #5
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    Thinking about this, the plausible scenario where an RCD might find a use for itself is where two bits of gear are running off a generator sitting on a truck. One unit, say an urn heating up water for coffee/tea has an earthed housing and is sitting on the ground, the other unit is a work light with a cable that's been pulled back out of its grommet exposing a bit of bare wire. If someone grabs the light and their finger touches what turns out to be an exposed active, there's now a path through them to ground, to the urn's body, back through its earth wire to the generator and then to neutral.

    I ran into this quandary when looking at the correct way to earth a 240V inverter on a mobile cherrypicker. In that case, the decision comes down to how many outlets there are. If there's a single outlet in the basket, then running the inverter with no earth link to the frame and no RCD was OK, but if there were two outlets, you then introduce the possibility of an earthed housing creating a potential active-human-earth path.

  6. #6
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    Quote Originally Posted by simonl View Post
    Hi all,

    I have always wondered how the following system works;

    At work we have a generator installed in the truck. It's not installed permanently or electrically, just has a dedicated spot and held in place with an appropriate tie down. In use, we can either use it in the truck or remove it and place it on the ground. Either way, it is electrically isolated from the truck and the ground. We run waterproof extension leads and lighting towers for scene lighting (although these are being phased out with ELV LED lighting)

    The extension leads run to a distribution board which has built-in RCD protection. My question is this;

    Since the generator has no specific grounding wire from the chasis to the ground, even if there was an earth leakage from any part of the circuit to the ground, no current would flow anyway and no one should ever be electrocuted since there is no current path from the ground back to the generator. Why do we hav an RCD installed AND how can it work?

    Sure, we could add an earth stake from the generator to the ground but we dont and that would just make the system less safe and then we would really need the RCD?

    Simon
    Do you have access to the instruction manual for your gennie? I will nearly guarantee you that it will state that an earth stake must be used with grounded equipment. There will be an earth stud on the gennie somewhere to connect it. Hardly anybody does it and we all should, particularly when running non double insulated gear such as welders. The possibility of items other than scene lighting being plugged in needs to be considered as well, (PPV fan for example).
    I am guessing to work for MFB and would therefore be running a Honda Gennie (most emergency services seem to). You can find the manual for your gennie here. http://about.hondampe.com.au/Owners_Manuals
    Secondly,is would not hurt to pose the question to your HSR or safety committee.

  7. #7
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    Quote Originally Posted by Karl Robbers View Post
    Secondly,is would not hurt to pose the question to your HSR or safety committee.
    Yea, I'm the HSR at our particular station so this is why I'm interested.

    You hit the nail on the head, it's a Honda EU series generator. Hardly gets used but non the less it's still available for use on the truck. Our PPV is petrol driven (again a honda GX150) but we will soon be receiving upgraded equipment including battery rescue cutting tools, a battery driven PPV and LED scene lighting which I am looking forward to; However, these generators will not be simply thrown out, they will no doubt be hand me downs to other quieter volunteer stations so the potential issues still exist.

    Thing that puzzles me (at this stage) is that all our gear and trucks go through such a riggorous development process including consultation with many, many working parties (some I assume would have sparkies in previous jobs) and union consultation. So i'm not sure how it's been overlooked.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  8. #8
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    Quote Originally Posted by simonl View Post
    Yea, I'm the HSR at our particular station so this is why I'm interested.

    You hit the nail on the head, it's a Honda EU series generator. Hardly gets used but non the less it's still available for use on the truck. Our PPV is petrol driven (again a honda GX150) but we will soon be receiving upgraded equipment including battery rescue cutting tools, a battery driven PPV and LED scene lighting which I am looking forward to; However, these generators will not be simply thrown out, they will no doubt be hand me downs to other quieter volunteer stations so the potential issues still exist.

    Thing that puzzles me (at this stage) is that all our gear and trucks go through such a riggorous development process including consultation with many, many working parties (some I assume would have sparkies in previous jobs) and union consultation. So i'm not sure how it's been overlooked.

    Simon
    As a fellow HSR, I can say that these things are missed quite easily. The small details are the ones usually missed. I'd love to see a battery PPV and YES! the new battery operated rescue gear is sexy!!!
    I have yet to see a gennie with the earth stud connected to anything unfortunately, even during my time working in the mining industry. I don't know how MFB would take the question being posed, but it could put the cat amongst the pidgeons.

  9. #9
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    Quote Originally Posted by Karl Robbers View Post
    As a fellow HSR, I can say that these things are missed quite easily. The small details are the ones usually missed. I'd love to see a battery PPV and YES! the new battery operated rescue gear is sexy!!!
    I have yet to see a gennie with the earth stud connected to anything unfortunately, even during my time working in the mining industry. I don't know how MFB would take the question being posed, but it could put the cat amongst the pidgeons.
    I actually work for CFA, however both MFB and CFA have very similar gear. Also, in a few months I suspect both MFB and CFA fulltime staff will be combined into a new fire service.

    People in these committies and working parties, along with OH&S work groups and usually very proactive and are receptive to new ideas or oversights, so that should not be an issue.

    Edit: yes, the new battery operated gear is great. Can't wait to get it. This will convert our truck to a "light rescue" meaning we can perform extrication (cut outs) of trapped people in car accidents. At the present we sometimes wait on scene for 15 minutes for the rescue to arrive. So it will be a game changer.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  10. #10
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    Quote Originally Posted by Michael G View Post
    Not being an electrical person, I'm not sure whether I am correct in this or not but here goes -

    In 'normal' use (everything is working correctly), the earth does nothing. Current circulates from active to neutral and so on. The theory behind the earth is that if there is a fault, the current can flow to 'earth' rather than through a person touching the faulty item. A person's resistance is higher. Without that earth wire, all the current would flow through the person. With the earth (it having lower resistance), current flows mainly through the path of least resistance.

    RCD's rely on the current in being the same as the current out - that is, current through the active should be the same as current through the neutral. Normally if there is a fault to earth, the RCD 'sees' the in-balance and trips.

    On a device without a (low resistance) ground connection, an RCD should still be operable as the earth/ ground has nothing to do with what the RCD monitors. All it needs to operate is for current to be leaking away from it's correct path. While there may not be a proper ground connection, I would not wish to bet my life that the current could not find an escape path to ground it if wanted one. I would suggest that on un-grounded equipment, a functioning RCD is more important as in the absence of the earth connection, the only path for stray current is through a person, and while a potential circuit may not be there all the time, sooner or later one will be established.

    Michael
    Its not too often that I would disagree with Michael, but on this occasion I do. If there is no circuit, I can't see how the current could escape to ground, and if there is no connection from the earth terminal on the Honda alternator to a ground stake, and the alternator is isolated from the ground, there is no return path. If you did touch a bare active wire, but at the time you had no contact with the neutral, you would just be floating at 240 volts, just as a bird on a power line does. I am unaware if the neutral is grounded on a portable alternator or not, but I would assume that if they have an earth connection point, then that point would be connected to one side of the output from the alternator, or in the case of an EU series Honda, or similar inverter alternator, the neutral from the inverter would be connected to the grounding point, otherwise there would be no point that I can see in having it.
    As Michael and others have said, an RCD works by detecting out of balance currents and trip a breaker if one is detected, so in order for it to work, a current must pass through it from the alternator, but return via an alternate pathway back to the alternator. The alternate circuit is normally via the ground or earth, on a mains system, and for it to work with a Honda Eu alternator, it would need some kind of earth connection, with one side of the output grounded to that connection. If you contact both the active and the neutral lines from a supply with an RCD installed, you have no protection if there is no pathway for some of the current (often about 30 mA) to bypass the RCD on the return path to the alternator. You will be just part of the electrical load, and will be fried if there is no intervention.
    Well that is how I understand it, but for those who may not know, I am NOT an electrician, just a retired rigger, so my knowledge of things electric certainly has limits, I have been known to be wrong before, and no doubt will be wrong again, in fact my six year old grandson has already expressed his doubts.

  11. #11
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    I described in my post #5 above where an otherwise isolated generator can easily become un-isolated if a bit of gear with an earth connection to its conductive metal casing is placed on the ground. At that point the generator's earth (and by extension, neutral, as I'm reasonably sure they have a N-E link) is connected to ground in the same manner as if you'd run an earth stake (although with most likely higher impedance).

  12. #12
    BobL is offline Member: Blue and white apron brigade
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    RT makes a good point in relation to an isolated genset.
    It's all too easy to assume the Neutral is always connected to ground somewhere back along the circuit.
    FWIW I just checked my small 10+ year old Honda genny and there is >2MΩ between neutral and ground socket points.

  13. #13
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    My response was shaped by the idea that there was not necessarily a circuit all the time, but with a casual action, there could be. Related but not, today we were trouble shooting a welder. As well as welding badly, it was giving tingles (from the HF start) to people. We found a hole in the gas hose, and once that was patched, welding performance returned to normal. The tingling seems to have stopped too. With the arc not conducting through ionised gas, was it shorting through the next available path? The point of this rambling story is that we can usually predict the obvious, but when something malfunction, all sorts of things can happen.
    Just think - if that genny was sitting on a damp patch of ground or someone lent a shovel against it while picking something up, there could be a (bad) earth...

    Michael

    PS I am recommending the welder go to a specialist to be checked...

  14. #14
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    Quote Originally Posted by BobL View Post
    RT makes a good point in relation to an isolated genset.
    It's all too easy to assume the Neutral is always connected to ground somewhere back along the circuit.
    FWIW I just checked my small 10+ year old Honda genny and there is >2MΩ between neutral and ground socket points.


    I thought in your house neutral is connected to ground simply to complete the circuit back to the power station.

    A DC to AC inverter has no grounded neutral.
    Gold, the colour of choice for the discerning person.

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    Quote Originally Posted by Michael G View Post
    With the arc not conducting through ionised gas, was it shorting through the next available path?
    I think so... a couple of times I've started TIG welding having forgotton to connect the ground return lead and been shocked through my gloves/jacket via the workpiece by the HF start.

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