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View Full Version : How do earth leakage (RCD) devices work with generators mounted in a truck



simonl
13th Jan 2019, 07:13 PM
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

Michael G
13th Jan 2019, 07:38 PM
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

BobL
13th Jan 2019, 07:40 PM
[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.

simonl
13th Jan 2019, 08:31 PM
Yes, I see your point.

Thanks.

Simon

RustyArc
13th Jan 2019, 10:55 PM
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.

Karl Robbers
15th Jan 2019, 09:05 AM
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.

simonl
15th Jan 2019, 12:07 PM
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

Karl Robbers
16th Jan 2019, 02:40 AM
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.

simonl
17th Jan 2019, 07:44 AM
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

Ropetangler
17th Jan 2019, 03:33 PM
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.:):rolleyes:

RustyArc
17th Jan 2019, 03:44 PM
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).

BobL
17th Jan 2019, 04:12 PM
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.

Michael G
17th Jan 2019, 07:14 PM
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:U, 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...

.RC.
17th Jan 2019, 09:04 PM
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.

Pumpkinate
17th Jan 2019, 09:13 PM
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.

Ropetangler
17th Jan 2019, 10:57 PM
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.
As I understand it, your house wiring is isolated from the power station by the transformers between them, so the neutral goes between your home and the transformer supplying it. At both the transformer and at your home, the neutral leg will be grounded, so if a fault occurs where an active wire contacts the body of an appliance, the current path will be to ground via the earth wire, in this case it would be a virtual short circuit and would hopefully blow the fuse. This would protect the appliance from further damage, and anyone from the danger of electrocution if contacting the live appliance body. If the device was supplied via an RCD, then any current to earth either via the earth wire or an unfortunate operator of the device, would unbalance the forward and reverse currents in the RCD, so causing it to trip and disconnect the supply. I have no idea what the arrangements for the High voltage wiring is, perhaps the power station alternator has its star point grounded, or perhaps not, I simply have no idea, nor do I know what the setup is with all the intermediate transformers is, but the final one always has its neutral grounded as far as I know.
Just on the situation of an otherwise isolated from earth alternator becoming earthed when sitting on damp ground or if a machine connected to it was similarly situated on the ground, while I agree that it is possible for it to become partly earthed for want of a better term, I would point out though that most modern portable 240 volt alternators such as the Honda EU series machines are encased in plastic covers and have rubber feet, as would a considerable number of tools that get plugged into them, so I think on balance that mostly any earthing effect would be pretty marginal, and it would generally be a pretty high impedance connection to earth. I will check our EU20i tomorrow if I get a chance to see if it has a N-E link, but it seems that BobL has a 2MΩ impedance between neutral and earth points on his Honda genny, so it would seem that a good few do not have that link. If that is the case then there would not be much point using an earth stake without first making a change and grounding the neutral, if you want to get protected by using an RCD device.

jhovel
17th Jan 2019, 11:19 PM
Every RCD has a test button. If I understand correctly how it works, that button connects active to ground/earth conductor via a resistor to divert 30mA or so out of the circuit. THat is intended to trip the RCD as there is now a fault current not returning via neutral.
If the genset and attached equipment is not earthed. then this test button should not be able to divert any current and the RCD should not trip.
Simon, could you test tis next time you set the genset up? I would be interested in the context of learning. Next time I have my little genny connected to the campervan, I'll also test the RCD.

Here is another related curly one: In my shed, if I make a connection between NEUTRAL and earth, the RCD trips.... I cannot measure a voltage between them, suggesting that it should not be possible to have a current flow.... There is no connection between neutral and earth at the shed, but there is at the house - 30m away. Any guesses? Sending 30mA from active to earth WILL of course trip the RCD as well....

BobL
18th Jan 2019, 08:55 AM
Here is another related curly one: In my shed, if I make a connection between NEUTRAL and earth, the RCD trips.... I cannot measure a voltage between them, suggesting that it should not be possible to have a current flow.... There is no connection between neutral and earth at the shed, but there is at the house - 30m away. Any guesses? Sending 30mA from active to earth WILL of course trip the RCD as well....

This is normal is the circuit is is carrying even a small load.

When you say zero V between N and E do you mean 0.0000000V and were you doing the measurement at the switch board or a GPO?

With just the lights on in my shed (~2A in the shed A-N circuit) with a GPO switch on I measure 0.17V between N and E at the GPO, and interestingly a fluctuating 0-2V with the switch off.

If 30 mA is needed to trip the RCD, using Ohm's law the highest R needed between N and E to prevent RCD tripping at 0.17V is ~6Ω and I any N-E resistance should not be this high even if the N-E connections is 30 m away.

A good E-N connection should be <1Ω so for a 30mA current to flow you only need 0.03V between N & E (with the GPO switch on) to trigger the RCD.

At the GPO, with the switch off, the N-E resistance at the GPO is >2MΩ because the GPO switch disconnects the N from the GPO outlet pins, and I can't measure the resistance with the switch on because the small 0.17V AC signal on the N effects the meter.

The resistance of a human body between a fingertip and a foot (ie goes past your heart) is nominally a couple of thousand ohms so at 240V that pathway will 120mA, currents as low as 80mA can kill a healthy human and even 50mA can cause problems. I were to touch the N-E with the fingers of opposite hands with 0.2V on the N live circuit above the current through me would be 0.2mA and should not trigger the RCD.

RustyArc
18th Jan 2019, 10:26 AM
Every RCD has a test button. If I understand correctly how it works, that button connects active to ground/earth conductor via a resistor to divert 30mA or so out of the circuit.

An RCD unit does not have an earth connection, just A & N connections. The test button places a resistor between the active and neutral on opposite sides of the unit (i.e. load active to supply neutral).

Back to generators though. If there is no N-E link on the generator, I'm not sure how an RCD is ever going to trip?

.RC.
18th Jan 2019, 12:35 PM
An RCD unit does not have an earth connection, just A & N connections. The test button places a resistor between the active and neutral on opposite sides of the unit (i.e. load active to supply neutral).

Back to generators though. If there is no N-E link on the generator, I'm not sure how an RCD is ever going to trip?

ecause it measures power going in and power going out. If there is a discrepancy it shuts the circuit down.
B

RustyArc
18th Jan 2019, 01:44 PM
ecause it measures power going in and power going out. If there is a discrepancy it shuts the circuit down.

Sure, but if there is no link between neutral and earth, then there is no other return path from active except for neutral, hence the RCD will never see a discrepancy and will never trip.

russ57
18th Jan 2019, 11:41 PM
But unless your appliances are all double insulated, they will have a 'chassis/ground' which is returned to the 'chassis/ground' of your generator truck, via the 'earth' wire of the supply cord.
So there is an alternative return path, although to be fair there is maybe a slightly lower probability of a fault. And of course your appliance could easily be connected to both the truck earth and the 'real' earth eg by sitting on the ground.


Russ

RustyArc
19th Jan 2019, 09:20 AM
But unless your appliances are all double insulated, they will have a 'chassis/ground' which is returned to the 'chassis/ground' of your generator truck, via the 'earth' wire of the supply cord.
So there is an alternative return path, although to be fair there is maybe a slightly lower probability of a fault. And of course your appliance could easily be connected to both the truck earth and the 'real' earth eg by sitting on the ground.

If there is no N-E link, then while there's a possible path along the earth wire, chassis and all that, there simply can't be a circuit as the earth isn't connected to neutral, hence active and earth have no potential between the two.

I'm going to have to do a bit of checking on the earth and neutral pins of the inverter and generator I've got access to...

russ57
19th Jan 2019, 10:17 PM
That makes sense

It would behave like an isolation transformer in that case.

Interesting...


Russ

eskimo
22nd Jan 2019, 08:49 AM
ecause it measures power going in and power going out. If there is a discrepancy it shuts the circuit down.
B

ine does this from time to time...why????does it drop the M below the top line or in RC's case the B..I'm glad its just not me...apologies for the hi jack on such an important topic.
M

BaronJ
23rd Jan 2019, 03:28 AM
Hi Guys,

It doesn't seem to be a browser issue because it only seems to happen on these forums.
My browsers are Firefox and Chrome ! I do have another browser, Konqueror, but I've never used it to browse the internet.

RustyArc
23rd Jan 2019, 09:38 AM
There's a lot of kookiness on this forum - the text editor in the Reply section often doesn't allow standard Windows behaviours, but it seems to be random.

I also received two email notifications yesterday for replies to threads that didn't actually appear in the thread itself.

Edit - for those last two, the posts have appeared - I'm guessing moderation has been turned on for those threads (or posters) delaying their post appearing until approved.