Safety when using a 240V mains inverter in a field or car park

[tggzzz]

OK, time to show my ignorance.

I sometimes sell working equipment at hamfests. They are easy to demo at indoor sites where mains power is available, but could they also be demoed at outside venues?

What I'm considering is:

• some equipment where the case is normally earthed, e.g. scopes
• some equipment where the case may not be earthed
• on tarmac or on grass (damp or dry)
• not in rain!
• power source: a car battery, probably in a car separated from the earth by rubber tyres
• 240V pure sine wave inverter

Obviously there is no standard protective mains earth connection available, so what can be done and must be done to ensure people don't get an electric shock?

Pointers to standard theory/practice information sources or to search terms would be welcomed.

[Bratster]

Get a decent quality inverter that has a GFCI/rcd outlet on it.

Sent from my Pixel 2 XL using Tapatalk

[tggzzz]

Get a decent quality inverter that has a GFCI/rcd outlet on it.

Thanks.

Is a GFCI/RCD sufficient to ensure safety in these circumstances?

Apart from that, I'm not sure how to tell, before purchase, whether an inverter would be indecent

[Zero999]

The mains earth should be connected to the car chassis. Most car inverters connect the neutral and earth connections to the 0V of the 12V, which will be connected to the vehicle chassis anyway, so there's generally no need to do it separately.

Yes an RDC/GFCI is essential to protect against shock.

[tggzzz]

The mains earth should be connected to the car chassis. Most car inverters connect the neutral and earth connections to the 0V of the 12V, which will be connected to the vehicle chassis anyway, so there's generally no need to do it separately.

Yes an RDC/GFCI is essential to protect against shock.

Do cars have HT (i.e. vaguely 240V) inverters nowadays? The last car where I worked on its electrics eventually had a fuel guage calibrated in Reontgens/hr

I'm happy that an RCD is necessary, but is it sufficient - or are there other precutions that should be taken?

[Zero999]

The mains earth should be connected to the car chassis. Most car inverters connect the neutral and earth connections to the 0V of the 12V, which will be connected to the vehicle chassis anyway, so there's generally no need to do it separately.

Yes an RDC/GFCI is essential to protect against shock.

Do cars have HT (i.e. vaguely 240V) inverters nowadays?

I was referring to the type of after-market inverter designed to be used in a car. They are normally connected via a cigarette lighter plug.

[IanB]

I'm happy that an RCD is necessary, but is it sufficient - or are there other precutions that should be taken?

Why is there potentially more danger than exists every day around the home?

I assume you will have normal insulated BS1363 sockets and plugs for the equipment. I assume you will not have frayed mains cables or bare mains wiring anywhere. I assume the equipment under inspection will either be earthed or double insulated. I assume people won't be disassembling or poking around inside equipment while it is plugged in. I assume you won't be working outside in the rain.

I am puzzled. Where do you think the danger lies?

[themadhippy]

Take a look at IT earthing arrangements.

[capt bullshot]

I'm happy that an RCD is necessary, but is it sufficient - or are there other precutions that should be taken?

I don't get it, why should an RCD considered necessary in such kind of setup?
Supposed you're using a decent quality inverter, the risk of electric shock is lower than your normal setup at home, because the inverter is supposed to provide a floating (isolated from any kind of ground or other reference potential) output. So if you touch one of the output pins (no matter if it's labeled live or neutral), no current will flow - no electric shock for you. Same for a possibly defective piece of equipment that shorts one of the mains input wires to its metallic case - no current flow - no electric shock due to the isolated output of the inverter.
So, a second fault (the other wire has to be shorted to a "real" ground) has to occur while the first fault exists to make your setup dangerous. For this reason professional setups that have an isolated source (look up IT mains) must have some kind of earth fault detection, but for an temporary setup this isn't considered necessary. Any kind of "home standard" RCD / GFCI won't do anything useful in such kind of setup, you'd need a special isolation fault detector made for IT mains setups - such a thing will cost a multiple of a decent quality inverter.
Just don't use crap inverters.

[tggzzz]

I'm happy that an RCD is necessary, but is it sufficient - or are there other precutions that should be taken?

Why is there potentially more danger than exists every day around the home?

I assume you will have normal insulated BS1363 sockets and plugs for the equipment. I assume you will not have frayed mains cables or bare mains wiring anywhere. I assume the equipment under inspection will either be earthed or double insulated. I assume people won't be disassembling or poking around inside equipment while it is plugged in. I assume you won't be working outside in the rain.

I am puzzled. Where do you think the danger lies?

None of the equipment will be earthed per se; it will nominally be floating. But I know that I don't know about the corner cases.

For example, I've repeatedly and reliably had minor electric shocks from a piece of metal that was visibly insulated from everything else by plastic. The person that put the equipment together denied that I could be getting any shock, but refused to touch it himself! Eventually we decided that the metal was a capacitor plate floating at half mains voltage. Adding an earthing wire that bypassed the plastic hinges and catches solved the problem.

I also know that many of the regulations are strange when read in isolation, but they reflect a solution to problems that have occurred in the past. If the problem and solution is understood, then the regulations make sense.

[tggzzz]

I'm happy that an RCD is necessary, but is it sufficient - or are there other precutions that should be taken?

I don't get it, why should an RCD considered necessary in such kind of setup?
Supposed you're using a decent quality inverter, the risk of electric shock is lower than your normal setup at home, because the inverter is supposed to provide a floating (isolated from any kind of ground or other reference potential) output. So if you touch one of the output pins (no matter if it's labeled live or neutral), no current will flow - no electric shock for you. Same for a possibly defective piece of equipment that shorts one of the mains input wires to its metallic case - no current flow - no electric shock due to the isolated output of the inverter.
So, a second fault (the other wire has to be shorted to a "real" ground) has to occur while the first fault exists to make your setup dangerous. For this reason professional setups that have an isolated source (look up IT mains) must have some kind of earth fault detection, but for an temporary setup this isn't considered necessary. Any kind of "home standard" RCD / GFCI won't do anything useful in such kind of setup, you'd need a special isolation fault detector made for IT mains setups - such a thing will cost a multiple of a decent quality inverter.
Just don't use crap inverters.

See my previous response.

Fundamentally the graveyards and law courts are littered with people that believed they understood the risks, but didn't understand odd corner cases. As an illustration, there are other threads on this forum concerning how you should and shouldn't use the combination of isolation transformers and RCDs. People were fine - until they weren't.

The concept of a special isolation fault detector made for IT mains setups may be useful.

[capt bullshot]

Fundamentally the graveyards and law courts are littered with people that believed they understood the risks, but didn't understand odd corner cases. As an illustration, there are other threads on this forum concerning how you should and shouldn't use the combination of isolation transformers and RCDs. People were fine - until they weren't.

I do agree in general, for the corner cases and darwin award candidates - look at my signature

The concept of a special isolation fault detector made for IT mains setups may be useful.

This is a company that makes such stuff:
https://www.bender.de/en/products/insulation-monitoring-overview
(not associated to them, just the first that comes to my mind)
These things usually are made to alarm you of the fault, but don't shut down the system since a single failure isn't dangerous in the first place. So depending on the situation, you'll have enough time to finish a critical job, do an ordered shut down, repair and start again.

[tggzzz]

Fundamentally the graveyards and law courts are littered with people that believed they understood the risks, but didn't understand odd corner cases. As an illustration, there are other threads on this forum concerning how you should and shouldn't use the combination of isolation transformers and RCDs. People were fine - until they weren't.

I do agree in general, for the corner cases and darwin award candidates - look at my signature

The concept of a special isolation fault detector made for IT mains setups may be useful.

This is a company that makes such stuff:
https://www.bender.de/en/products/insulation-monitoring-overview
(not associated to them, just the first that comes to my mind)
These things usually are made to alarm you of the fault, but don't shut down the system since a single failure isn't dangerous in the first place. So depending on the situation, you'll have enough time to finish a critical job, do an ordered shut down, repair and start again.

Thanks for the ref; I'll investigate.

As for your .sig, I do things (and have encourged my daughter to do things, e.g. flying solo and making "forced landings" before she could drive a car, backpacking around India) that others consider dangerous. Fundamentally what we do to ourselves is our business. But I object strenuously when other people endanger me, since that doesn't affect their ability to reproduce

[Zero999]

Ultimately all metalwork needs to be electrically connected together, to ensure it's at the same potential. The safety/protective "earth" on the mains circuit needs to be connected to the car's chassis to ensure that the two can't float at different voltages, which could present a shock hazard.

An RCD/GFCI are essential if this is going to be operated in a relatively damp environment. I know the original poster said they won't use it when it's raining, but being outdoor, especially in a wet climate such as the UK, does increase the chance of water ingress, over being indoors. Note that an RCD will only be of any use if the neutral of the inverter is tied to protective earth.

[IanB]

None of the equipment will be earthed per se; it will nominally be floating. But I know that I don't know about the corner cases.

Since independent mains supplies are common (fairgrounds, public events, outdoor concerts, etc.) I am sure there are guidelines and regulations about how to set things up safely. They would be a good reference.

If I were thinking about the problem, I would first make sure all exposed metalwork is equipotentally bonded and call this the system earth/ground. This would mean the car chassis and everything called "earth" in the supply through the third pin of the mains plugs. Then I would tie the "neutral" wire of the inverter to this system ground so that the line voltage can never be more than 240 V away from system ground (ensuring it cannot float freely). Lastly I would install an RCD downstream of the earth/neutral tie, so that the RCD will trip if any current takes a different return path outside the designated neutral wire (e.g. if someone touches the car chassis at the same time as the 240 V line).

This is just my supposition about how I might go about things. I am not an expert, so I don't know about corner cases either. This is where authoritative published guidelines would come in helpful.

[IanB]

For example, I've repeatedly and reliably had minor electric shocks from a piece of metal that was visibly insulated from everything else by plastic. The person that put the equipment together denied that I could be getting any shock, but refused to touch it himself! Eventually we decided that the metal was a capacitor plate floating at half mains voltage. Adding an earthing wire that bypassed the plastic hinges and catches solved the problem.

There is an interesting observation to be made here about which power distribution or transmission lines are birds willing to sit on. For example, birds happily sit on 11 kV and 33 kV lines with no concern at all. I have occasionally (I think) seen birds sitting on 132 kV lines. I don't particularly recall seeing birds sitting on 275 kV or 400 kV lines. So there is some voltage where capacitive currents become an issue. If birds can sit on high voltage distribution lines, I feel a human should be able to touch a 240 V mains wire without discomfort (indeed, I have done this many times during my reckless youth and not even realized I was touching a live mains wire until afterwards). When you are a kid it is easy to forget to unplug things before working on them. (I do not recommend being young. It is a very dangerous time.)

[tggzzz]

For example, I've repeatedly and reliably had minor electric shocks from a piece of metal that was visibly insulated from everything else by plastic. The person that put the equipment together denied that I could be getting any shock, but refused to touch it himself! Eventually we decided that the metal was a capacitor plate floating at half mains voltage. Adding an earthing wire that bypassed the plastic hinges and catches solved the problem.

There is an interesting observation to be made here about which power distribution or transmission lines are birds willing to sit on. For example, birds happily sit on 11 kV and 33 kV lines with no concern at all. I have occasionally (I think) seen birds sitting on 132 kV lines. I don't particularly recall seeing birds sitting on 275 kV or 400 kV lines. So there is some voltage where capacitive currents become an issue. If birds can sit on high voltage distribution lines, I feel a human should be able to touch a 240 V mains wire without discomfort

Indeed, I have a similar intuition. But I did about that piece of metal too!

Then we consider people with pacemakers, or undiagnosed dicky tickers

If I was being spectacularly cruel, I might comment that losing the odd old radio ham might also go unnoticed - except for the paperwork.

(indeed, I have done this many times during my reckless youth and not even realized I was touching a live mains wire until afterwards). When you are a kid it is easy to forget to unplug things before working on them. (I do not recommend being young. It is a very dangerous time.)

We survived it, by definition.

I remember lightly touching live with one hand and neutral with the other. Fortunately the uncontrollable biceps contraction broke the circuit.

Such memories, plus watching people die in accidents, make me less sanguine about the unexpected.

[tggzzz]

None of the equipment will be earthed per se; it will nominally be floating. But I know that I don't know about the corner cases.

Since independent mains supplies are common (fairgrounds, public events, outdoor concerts, etc.) I am sure there are guidelines and regulations about how to set things up safely. They would be a good reference.

If I were thinking about the problem, I would first make sure all exposed metalwork is equipotentally bonded and call this the system earth/ground. This would mean the car chassis and everything called "earth" in the supply through the third pin of the mains plugs. Then I would tie the "neutral" wire of the inverter to this system ground so that the line voltage can never be more than 240 V away from system ground (ensuring it cannot float freely). Lastly I would install an RCD downstream of the earth/neutral tie, so that the RCD will trip if any current takes a different return path outside the designated neutral wire (e.g. if someone touches the car chassis at the same time as the 240 V line).

This is just my supposition about how I might go about things. I am not an expert, so I don't know about corner cases either. This is where authoritative published guidelines would come in helpful.

I agree with all of that.

Now, where can I find such authoratative (and preferably comprehensible) guidelines?

[Siwastaja]

Ultimately all metalwork needs to be electrically connected together, to ensure it's at the same potential. The safety/protective "earth" on the mains circuit needs to be connected to the car's chassis to ensure that the two can't float at different voltages, which could present a shock hazard.

You are not wrong, but I want to stir things up a little bit:

Note that what you describe is exactly the mechanism which makes it dangerous in the first place.

Floating is of course safe by definition - there is no path for the current. You need to literally get between the two "live" wires (L&N, which are not floating respective to each other), almost impossible to happen by accident as a consumer of products.

It's just that for large scale power delivery, attempts to have a floating system have failed or have been impractical. For many reasons unimportant here, they can't avoid grounding, so they need to ground properly. But grounding of the pipework, for example, is exactly what causes the deaths; contrary to popular misconception, it's not a safety feature against electrocution, it's the opposite of a safety feature. From the fact that the pipework needs to be grounded, comes the absolute requirement to ground metal cases of the devices to the same potential, so that if a live wire gets loose inside that equipment, instead of applying a voltage to the case, it blows a fuse, so that you can't accidentally touch the pipework and the case of the broken device at the same time, and receive a shock. If nothing was grounded, there wouldn't be an issue, either - this can be locally achieved with isolation transformers.

The nice idea was that by mandating (by law), that any device would have its metal parts connected to the same earth pin, such incidents would be prevented. But products do fail, and people fail to follow the regulations. So in some cases, the "protective earth" is a very actual deathtrap. The name is misleading.

It wasn't until about late 1990's this problem was finally fixed: by introducing RCDs. They completely changed the game, now the "protective earth" suddenly isn't a deathtrap anymore. Still some 30-40 years ago, dying of electrocution was, although rare, a real cause of death. Now it's approximately comparable to being caught by UFOs. RCDs play a big part in this change. (In this country, for example, the death rates went down from 20/year to 0/year within just two decades. During the same time, traffic accident fatalities went from around 800 to around 400.)

So by giving the advice of carefully grounding every metal piece, you are first creating the safety issue, which you are then solving by adding an active protection device. After all, with a car battery and an inverter, a truly floating system is a very actual possibility! So now we are back looking at the same issue they had to look when designing electric distribution and grounding. Can you have a floating system? Or do you need to ground, and then add protection against the risk it created? The answer should be in the user manual of the inverter. For example, if the inverter is not isolating at all, then you could receive a shock by being between a loose live wire inside a faulty device, and your car metal body. In which case, you need to tie the case of this faulty device to the metal body of your car, so that a short circuit current flows and blows your cheap inverter, instead of allowing electrocution. Or, even better, highly recommend, add the RCD. The answer should be in the manual, anyway.

By all means, use an RCD, it's one of the best things to happen since sliced bread. In a floating system, there just isn't any mechanism for it to act, because the type of fault it's protecting against cannot happen. But trusting a forum post deduction isn't always a wise thing to do, and if we miss something, the RCD can be valuable. It's not going to hurt (as long as you are able to prevent the "false sense of security" phenomenon).

All in all, if you buy certified, somewhat known brand products, and use them as intended, the risk of electric shock is almost zero today. This includes connecting an inverter to a battery and powering things with it. Regulation exist for a reason.

What comes to inverters, it is completely impossible to try to google the matter, because people, inverter manufacturers included, completely mix up the concepts of "grounding", with at least three totally different meanings in this context. The only relevant question here is, is there a path for current to flow from either of the L/N wires, to the battery input terminals. If there is, then for a safely floating system, the battery, and if it's in a car, the whole car itself, must be protected from touch by isolating material. But if the inverter isolates, it's trivial to build a floating system.

Forbidding the word "ground" by law would probably increase the safety more than any other mean.

[IanB]

Floating is of course safe by definition - there is no path for the current.

Question: What happens if there is some inadvertent charge pump mechanism that can raise the floating conductors to some very high voltage relative to the surroundings? In that case touching the floating system could lead to a static discharge that may be uncomfortable or even dangerous if the capacitance of the system is large enough. So is floating truly safe by definition? In some contexts (e.g. aircraft fueling) floating is very dangerous and must be avoided.

[tggzzz]

Floating is of course safe by definition - there is no path for the current.

Question: What happens if there is some inadvertent charge pump mechanism that can raise the floating conductors to some very high voltage relative to the surroundings? In that case touching the floating system could lead to a static discharge that may be uncomfortable or even dangerous if the capacitance of the system is large enough. So is floating truly safe by definition? In some contexts (e.g. aircraft fueling) floating is very dangerous and must be avoided.

Good counter example. One of my professors at uni was involved in understanding why aircraft were being destroyed during fuelling, and how to avoid it.

Also, that metal plate I referred to was floating; it was connected to everything else by plastic. Not dangerous per se, but not something to be allowed if it can be avoided.

[themadhippy]

Now, where can I find such authoratative (and preferably comprehensible) guidelines?

BS7671 is a good starting point,but at around £80 a tad on the pricey side

attempts to have a floating system have failed or have been impractical. For many reasons unimportant here,

A good start would have been running a decent lump of  copper  aluminum instead of saving a few quid and using mother earth as the return leg

[Siwastaja]

Floating is of course safe by definition - there is no path for the current.

Question: What happens if there is some inadvertent charge pump mechanism that can raise the floating conductors to some very high voltage relative to the surroundings? In that case touching the floating system could lead to a static discharge that may be uncomfortable or even dangerous if the capacitance of the system is large enough. So is floating truly safe by definition? In some contexts (e.g. aircraft fueling) floating is very dangerous and must be avoided.

Exactly this is the reason large-scale floating systems didn't work out. Lightning protection could be also mentioned, but it's a special case of what you just described (static charge accumulation and release).

[HackedFridgeMagnet]

....

Forbidding the word "ground" by law would probably increase the safety more than any other mean.

Nice thoughtful post btw.

But the last sentence, I don't get it?

[Siwastaja]

....

Forbidding the word "ground" by law would probably increase the safety more than any other mean.

Nice thoughtful post btw.

But the last sentence, I don't get it?

Whenever the term "ground" is involved, people start to talk about completely random things, not related to the subject. It becomes a problem when the people who think in terms of "feeling" words like "ground", start to implement the circuits.

The issue is that "ground" abstracts too much. It works in simple cases like a synonym for "reference voltage plane", for example, in a electronic circuit where there is one shared return path for all currents contained within the system.

But as soon as you have things like "protective earth", "mains neutral", ground planes of PCBs, battery negative, and so on, in a complex system involving all of them at the same time, people who have no idea start conveniently call any of these "ground".

Ground is almost never a proper terminology when it comes to mains wiring. There is a huge difference between PE and N, and by talking about "ground", people may arbitrarily mean whatever, or maybe just don't know about the difference.