Should a giant steel framed human aquarium be earth bonded?

[Deuce]

I have a large, portable steel framed tank. approx 6m x 2m x 2m. It is used for actors/models to perform in for a variety of productions.

For many years now, I have had conflicting advice about whether or not it should be earth bonded. The initial response is "it's a giant steel structure, bond it!!". But then invariably someone else comes along and points out that, should the tank itself become energised, the people in the water within the tank would become part of the return to earth - the current would flow through the tank and them. If it was not bonded, as it is by design insulated, they would be ok. At least until they tried to leave the tank.

To date our solution has been to be super cautious of anything that could even present the possibility of bringing current to the tank itself. But how do we make a once and for all decision on whether it should be bonded without any professional consensus of opinion?

 

[Leesparkykent]

I have a large, portable steel framed tank. approx 6m x 2m x 2m. It is used for actors/models to perform in for a variety of productions.

For many years now, I have had conflicting advice about whether or not it should be earth bonded. The initial response is "it's a giant steel structure, bond it!!". But then invariably someone else comes along and points out that, should the tank itself become energised, the people in the water within the tank would become part of the return to earth - the current would flow through the tank and them. If it was not bonded, as it is by design insulated, they would be ok. At least until they tried to leave the tank.

To date our solution has been to be super cautious of anything that could even present the possibility of bringing current to the tank itself. But how do we make a once and for all decision on whether it should be bonded without any professional consensus of opinion?

It would be site specific.

 

[chris gray]

Earthing and bonding as far as I’m aware as 2 separate things

 

[Deuce]

Earthing and bonding as far as I’m aware as 2 separate things

Maybe I'm wrong in my terminology. I thought bonding was the connection of two conductive systems, and in this case bonding the tank to earth = earth bonding. Feel free to educate me though - that's why I'm here asking

 

[Deuce]

It would be site specific.

Can you give me an example of a site where bonding the tank would not put those inside at risk if it were to become live?

We take site specific factors into account for a variety of reasons but I can't see how the tank being earthed would ever be a good thing for the people in the tank.

 

[Marvo]

Is Rcp ≥ (Uo/Ib)? What earthing arrangement does the electrical installation have?

 

[Deuce]

For clarity; my concern is that because the tank is full of water, it is the equivalent of forming a conductive shell around the occupants, and then passing current through it. It's not the same as earthing a fridge so that if the outer skin becomes live, anyone touching it doesn't carry the current. It's the equivalent of making them part of the earthed item itself.

 

[Deuce]

Is Rcp ≥ (Uo/Ib)? What earthing arrangement does the electrical installation have?

Varies! Our own kit (the filtration system) is bunded in case of leaks and all electrical fully isolated from the water passing through it. The environment alters as the tank is used at various studios and venues around the world.

 

[Marvo]

So the whole tank and its associated equipment is effectively portable? It plugs in? It's also used in a variety of building structures ie some will have metal building elements in reach of the tank others won't........ Lol, You might have mentioned this in the OP

 

[PEG]

Is this fixed or mobile? I would be tempted to do a risk assessment where it is filled,and keep all cable connections away from it. Well away.
In my opinion,depending on where it is sited,the least number of possible electrical connections to it - the better.

 

[markythesparky]

What is the tank sitting on/ in?

In the case of swimming pools, they are in or on the ground (earth). The idea behind pool bonding is that everything touchable is brought to the same potential and therefore minimises shock risk. TN-C-S earthing systems are not allowed due to the risk of a disconnected neutral and the consequence of mains potential within the pool area.

My view would be to TT it and have anything near it on a 30mA RCD.

 

[Deuce]

What is the tank sitting on/ in?

In the case of swimming pools, they are in or on the ground (earth). The idea behind pool bonding is that everything touchable is brought to the same potential and therefore minimises shock risk. TN-C-S earthing systems are not allowed due to the risk of a disconnected neutral and the consequence of mains potential within the pool area.

My view would be to TT it and have anything near it on a 30mA RCD.

The one constant is that wherever the tank goes, 30ma RCD is the standard.

The tank is always sat upon a butyl membrane, so it's isolated.

If it was TT'd, of course that is a sound path to discharge current. But if the clamp is at the base of the tank, and a live lamp collapses and hits the top of the tank, the current would flow down through the tank and the water and through the people inside it surely. In that scenario wouldn't it be better that it was left isolated?

 

[chris gray]

Surely bonding would create a path?

 

[Marvo]

So there's 230v lighting within zone 1? This just gets better....
Did the tank and its associated equipment come as a certified design or is it just something that was fabricated and that's evolved and been added to?

 

[Marvo]

Do you have some photos maybe?

 

[Deuce]

Surely bonding would create a path?

However it's connected to earth would create a path - the question is whether its safe to create a path through the tank.

 

[StephenRowley]

If the tank was bonded I doubt anyone in side it would see any current flow in fault condition as the current would find the path of least resistance to ground though the nice conducting water.

But really this needs someone who a expert in the field of swimming pool safety

 

[markythesparky]

The one constant is that wherever the tank goes, 30ma RCD is the standard.

The tank is always sat upon a butyl membrane, so it's isolated.

If it was TT'd, of course that is a sound path to discharge current. But if the clamp is at the base of the tank, and a live lamp collapses and hits the top of the tank, the current would flow down through the tank and the water and through the people inside it surely. In that scenario wouldn't it be better that it was left isolated?

The 30mA RCD is the key. It will disconnect the supply before the current gets to a dangerous level (50mA). If you didn't have the TT, then the tank would remain live and the RCD would not operate. That is until someone or something created the earth path.

Which do you think is safer?

 

[PEG]

Nice clean water is not a good conductor

I think the additional information,increases my belief that this needs a specialist,risk assessment.

Next,we will be told that these performers are lowered in this tank,using an electric crane,and i can think of half a dozen RA issues,on that fact,alone.

 

[Deuce]

Do you have some photos maybe?

 

[markythesparky]

If the tank was bonded I doubt anyone in side it would see any current flow in fault condition as the current would find the path of least resistance to ground though the nice conducting water.

But really this needs someone who a expert in the field of swimming pool safety

Don't run
Don't jump
Don't bomb
Don't spit
No petting
......

 

[PEG]

Don't run
Don't jump
Don't bomb
Don't spit
No petting
......

That's 5 out of 6 of my previous post's RA list

 

[Deuce]

The 30mA RCD is the key. It will disconnect the supply before the current gets to a dangerous level (50mA). If you didn't have the TT, then the tank would remain live and the RCD would not operate. That is until someone or something created the earth path.

Which do you think is safer?

You just opened up another sphere of debate we have had before. If isolated the performers remain safe, so long as they don't connect with the ground or some other earth point. So they will be ok if the fault is detected and power removed. However, if the tank is earthed they will definitely be subject to current until the RCD reacts, on the assumption that all RCD's are going to react - which isn't the case. It's a fact some have their breakdown moment after their most recent test.

 

[Marvo]

Don't run
Don't jump
Don't bomb
Don't spit
No petting
......

Just watched the video....Not sure we should rule out petting so quickly.....

 

[Deuce]

Nice clean water is not a good conductor

I think the additional information,increases my belief that this needs a specialist,risk assessment.

Next,we will be told that these performers are lowered in this tank,using an electric crane,and i can think of half a dozen RA issues,on that fact,alone.

To be honest all water is pretty crap as a conductor. "Clean" water isn't a conductor at all. The water in the tank is probably cleaner than your tap water, but will still conduct - albeit quite poorly.

 

[StephenRowley]

But the tank water I would hope have a chlorine or other water treatment in there. Which would massively increase its conductivity to better than that of human body.

What does the tank sit upon? It may all ready inadvertently have a path to ground though the ground it's self.

 

[Marvo]

I think the water conductivity is a moot point. This installation would be treated as a special location defined for bathrooms or swimming pools in the regs. I don't know the finer points of the UK regs but I'm inclined to agree with an earlier post suggesting a risk assessment and appropriate mitigation and management of risks be performed for every time it's used.

 

[Deuce]

But the tank water I would hope have a chlorine or other water treatment in there. Which would massively increase its conductivity to better than that of human body.

What does the tank sit upon? It may all ready inadvertently have a path to ground though the ground it's self.

It's dissolved solids, particularly minerals that increase conductivity. TBH I don't know to what extent chlorine does, but yes we use chlorine.

It sits on a 1mm rubber membrane.

It's not just about the water though. The performers could be in contact with the steel frame itself. Or the steel base of the tank.. Of course the steel is certainly a better conductor than us fleshy beings!! But adding the submersion in water of all parts does add an unusual angle to this problem.

 

[Strima]

Without seeing it set up and doing some testing we’re just stabbing in the dark.

 

[Deuce]

I suppose I should add, that clearly this is not a standard application or problem. It's a very unique thing - and the world is all the more richer for having such things.

My goal is to get to a point at which I'm confident that the best solution for safety is in place. As it stands, with the divide in opinion on this, that certainty isn't there. My personal view is that it should be earthed, as it minimises risk overall. Albeit, under certain unexpected conditions it could be said it would have been safer if it wasn't.

 

[Deuce]

Without seeing it set up and doing some testing we’re just stabbing in the dark.

What would you test for? I'm not asking if/how it could transmit current to earth, I'm asking if it should be allowed to in the first place.

 

[PEG]

I'm glad this has run to three pages...at last,i have some independent proof,why my 10 year old,cannot have a pet mermaid.

 

[Taylortwocities]

Sorry, couldn't resist this..

 

[Wilko]

Hi - just a couple of thoughts
- Has the installation been done in accord with BS7671 section 7 and BS7909 ?
- RA and full test prior to each use?

 

[littlespark]

I think we’re missing the big question here

Have you drowned anyone famous yet?

 

[Deuce]

Hi - just a couple of thoughts
- Has the installation been done in accord with BS7671 section 7 and BS7909 ?
- RA and full test prior to each use?

Hi,

The only part of section 7 which in spirit is tough to enforce is others placing/moving lamps closer to the tank than the regs would permit. I say "in spirit" as that requirement is specific to certain locations such as bathrooms, with specific rules for each defined special location.

Was there a particular requirement you were thinking of?

Regards BS7909, yes. That's the one that all event/studio sparks are well read up on. We don't have full control ourselves over what goes where in the layout of the temporary installations, such as the running of power cables and lighting in the area, but we do flag anything that is obviously a concern or may not have been given full consideration.

The sparks on site always ensure the following key areas:

- Every new electrical feed/cable is tested prior to being made available for connection, incl RCD

- All must be 30ma unless special equipment required a different solution.

- All third party electrical equipment is checked for testing within past 12 months and in general scrutinised for suitability of safety precautions.

- in potential wet areas (ie filming in a carpark where it could rain and water could pool) all portable distribution boxes are raised from the ground.

In general everything is considered, checked/tested ahead of each use, most of which last only a few days before being used for something else and being tested again. You could compare it to fully testing and reviewing the electrical installation in your bathroom everytime you take a bath.

The question of whether or not to earth the tank itself does remain a sticking point, but I can say sparks in charge of power on site do everything reasonable to make the surrounding electrical installation as safe as possible. They're 'on it'.

 

[johnduffell]

In terms of bs7671
Bonding - only if it can introduce a potential. The only way i can think is if it was sitting directly on the earth. In that situation the earth would be the extraneous conductive part.
Earthing - only if it is a part of the electrical installation designed to protect people from electricity eg class I appliance or fitting.

So it sounds like the answer is connection it to the main earthing terminal is like bonding a spoon.

If it's a temporarily installation other rules apply other than bs7671. In terms of common sender, to me, any conductive surfaces have a risk, and bonding them all just changes the risk to a different one.

 

[marconi]

Could you post ( I have only looked quickly over the previous posts so you may have done already) - a sketch of the set up including where the items of electrical equipment and their wiring are in relation(including distances) to the aquarium? Also, what voltage is the equipment and what is the electrical supply ie: generator, mains, TT/TNCS/TNS. Where is the water supply which fills it and include the pipework and whether metal or plastic?

 

[Wilko]

I can see why you've not reached a conclusion on this question. I can imagine setups where it may add safety, perhaps as performers enter and exit the tank and stepping onto GME while touching the tank etc. If you've a repeated system of installation (tank placement, access and egress, steps etc) that could be assessed perhaps and a conclusion reached. Personally, I want to TT the installation and connect all conductive parts, but it's not a cattle shed so I'm probably wrong

 

[marconi]

I can see why you've not reached a conclusion on this question. I can imagine setups where it may add safety, perhaps as performers enter and exit the tank and stepping onto GME while touching the tank etc. If you've a repeated system of installation (tank placement, access and egress, steps etc) that could be assessed perhaps and a conclusion reached. Personally, I want to TT the installation and connect all conductive parts, but it's not a cattle shed so I'm probably wrong

I have similar thoughts to you on this but still need to see a sketch - my early thoughts before seeing sketch are to make it a 55V-0-55V TT installation centre -tap earthed and the 110V passed through a 10mA double-pole RCD. 110V provided by a safety isolation transformer supplied by mains through a 30mA double pole RCD. Then bond all metalwork together and to terra-firma. OP - this is not a definitive answer.

 

[Strima]

I have similar thoughts to you on this but still need to see a sketch - my early thoughts before seeing sketch are to make it a 55V-0-55V TT installation centre -tap earthed and the 110V passed through a 10mA double-pole RCD. 110V provided by a safety isolation transformer supplied by mains through a 30mA double pole RCD. Then bond all metalwork together and to terra-firma. OP - this is not a definitive answer.

That was one of my initial thoughts but if you read further up there's reference to third party equipment such as lighting, with BS7909 being referenced that tells me this is a very temporary structure that could be used in numerous locations fed from varying supplies.

I would say at each location the installation should be assessed by the set sparky. You can take all the precautions you want but all it takes is a gust of wind to blow over a high level set light to completely balls things up.

 

[Deuce]

Could you post ( I have only looked quickly over the previous posts so you may have done already) - a sketch of the set up including where the items of electrical equipment and their wiring are in relation(including distances) to the aquarium? Also, what voltage is the equipment and what is the electrical supply ie: generator, mains, TT/TNCS/TNS. Where is the water supply which fills it and include the pipework and whether metal or plastic?

The usage scenario and layout, equipment is different at each use, and generally we arrive just ahead of use, and would start to fill the tank as soon as it's loaded into position (baring any obvious safety concerns that would prevent us doing so). Of course this is a very different way of working compared to what most sparks on here would consider reasonable - but that's TV/events. You hit the ground running.

The need for speed is the reason all studios and event teams have on site dedicated sparks to monitor/test/re-arrange in real time. And the sparks will live that life full time.

The water supply isn't really an issue as the tank is filled once, then disconnected from supply. In any case, the supply would be rubber/pvc hose of some form. The permanent pipework between the tank and it's filter system is PVC. The filter system itself is bunded to protect against internal leaks, with all electrical on the outside of the bund. And of course, RCD on board for each circuit, even though it would only ever be fed by a site feed that is also protected and tested prior to each use.

I think mostly in the UK at least exhibition/event venue power to each stand is TNS/TNCS. In studios normally TNS, and when we're in the middle of nowhere relying on generators, TT. Not much help huh?..

I have similar thoughts to you on this but still need to see a sketch - my early thoughts before seeing sketch are to make it a 55V-0-55V TT installation centre -tap earthed and the 110V passed through a 10mA RCD. 110V provided by a safety isolation transformer supplied by mains through a 30mA RCD. Then bond all metalwork together and to terra-firma. OP - this is not a definitive answer.

The industry is firmly on 240v, and this will not change. The specialist equipment needed doesn't exist in 110. In many ways 110v would probably make a great deal of sense as in reality, studios and exhibition venues represent about the most intensive construction environments you can imagine.

Everything else steel in these places will be connected to earth by default. That's certainly standard practice, it's just the tank which makes the sparks stop and reconsider sometimes. Hence my asking on here.

Whilst so much varies the constants are:

There will always be temporary 240v 30ma protected power routed all over the place around the space the tank is in, supplying hundreds of items of equipment for A/V, lighting, cameras, the list would be endless.

There will always be people entering/exiting the tank at times that the surrounding power supplies are needed to remain on.

People will always be in the tank at some point to perform in some way.

 

[Deuce]

That was one of my initial thoughts but if you read further up there's reference to third party equipment such as lighting, with BS7909 being referenced that tells me this is a very temporary structure that could be used in numerous locations fed from varying supplies.

I would say at each location the installation should be assessed by the set sparky. You can take all the precautions you want but all it takes is a gust of wind to blow over a high level set light to completely balls things up.

Indeed. anything can happen and not only is there limited time to plan ahead, the nature of the industry is that things can quickly evolve on site during testing/rehearsal.

The in house sparks or production sparks do indeed assess in advance as much as possible, and in real time throughout each production.

As for the set light falling into the tank scenario - yup, that's the obvious one. Albeit a whole team of very conscientious people make damn sure it won't happen. But it has to be assumed that one way or another there is a remote possibility something could cause the tank to become live, hence the ultimate question: are those in the tank at the time best protected by the tank being isolated or earthed?

 

[Wilko]

Hi - next question(s) how do the performers get in and out? And how is the water heated and or filtered?

 

[Deuce]

Hi - next question(s) how do the performers get in and out? And how is the water heated and or filtered?

Varies of course

I've gone into deals of the filter system further up, but it's essentially a swimming pool filter system, with swimming pool heaters in a bunded enclosure with all electrical on the outside of the enclosure.

Access is normally via platforms built by set construction. If there is any exposed steel it's typically connected to earth - although that's really the responsibility of the sparks and whoever has designed/built the structure. We just supply the tank and the filter system.

 

[static zap]

Have pondered this ,as occupants of a hot tub , reach ..Double insulated status ... but who knows with water around ..
__Some form of alarm of Risky potential - Should be available
( but this "Specialist situation is sure to be Expensive" )
If studio /stage lighting is not on RCD , I understand your reluctance to add an earth .
Should a light / cable break free and go fishing !

 

[Deuce]

Have pondered this ,as occupants of a hot tub , reach ..Double insulated status ... but who knows with water around ..
__Some form of alarm of Risky potential - Should be available
( but this "Specialist situation is sure to be Expensive" )
If studio /stage lighting is not on RCD , I understand your reluctance .
Should a light / cable break free and go fishing !

The hot tub analogy is a good one, and having modified a few I certainly never saw any method of earthing the water body directly, although the pump/heater would be earthed which would pass through to the water in the system. The same is true of the pump/heaters attached to the water body in my tank, so it too is already technically earthed - albeit a pretty poor connection (relatively clean water isn't really a sound way of making a connection).

I think the crux of the debate comes down to:

What is the risk of a none RCD protected piece of equipment falling in to an unearthed tank vs the risk of RCD protected equipment falling into an earthed tank.

Having seen literally millions of lights etc rigged at high level I can say I have never seen one fall. If one did fall, it would almost certainly break the power connection on the way down as the power cables are invariably threaded through long lengths of truss, or mechanically clamped in some way at a high level. And then as the "all on RCD" rule is so widely enforced, the chances of a light falling, the power cable reaching the tank in tact and not being RCD protected is incredibly small. It would truly be a freak incident.

The only other scenario is a mechanical failure of the tank itself, leading to instantaneous flooding of the area. That could contact power ahead of RCD in theory, but the grounding effect of water of such a large area would dissipate current very efficiently, not to mention it would be in contact with several other grounded steel structures - so long as no one tries to rescue the situation by picking a power cable...

So I think I have come to a conclusion, the tank should be earthed, that seems to be the majority view on here and on site. Even if a freak occurrence did result in a shock that wouldn't have happened had the tank remained isolated, it is still true to say that the greatest likely risk was considered and mitigated by earthing it.

Anyone who feels this thinking is still worth challenging please do speak up.

 

[Nigel]

This is quite an interesting thread.

As I see it there are differnet risks:

1. The risk of someone entering the pool and a potential difference between the pool and the ground.

This could be mitigated by ensuring the rubber matting you have provided at least a 1.5m overlap so that nobody could be touching true earth and your bonded pool.

2. Users inside the pool completely and equipment possibly bringing the water to mains potential.

This can be mitigated by using Class 2 equipment only or SELV only. Preferable the latter. An RCD plug on all your equipment also which would need to be tested periodically to ensure it works. I would not like to rely on the customers RCD protection not knowing the state of it.

Another option would be to provide an earth free equipotential bonding zone within the pool. This would mean all exposed conductive parts and extrenuous conductive parts would need to be bonded together so that they are at the same potential. The risk here is to people that are not in the pool and could touch the metal frame and true earth.

3. Users in the pool and able to touch electrical item outside of the pool that form a part of the building electrics.

This is an issue as you cannot really ask the customer to move their lights etc. So this would have to be surveyed and if any electrical equipment fell inside of the zones then you would have to decline that position of siting your pool.

As you can see there are so many things to consider each site will need to be thoroughly surveyed each time.

To be honest I would be inclined to suggest the following:

Pool only to be sited on insulated matting with a 1.5m overlap and so that no electrical iteams are within zones as per BS7671.

All electrical equipment to be Class 2 or SELV and be RCD protected by your own RCD plugs.

When people are in the pool all electrical items to be disconnected from the electrical supply and reinstated when all people are clear. Completely remove the risk when people are in there.

 

[Deuce]

This is quite an interesting thread.

As I see it there are differnet risks:

1. The risk of someone entering the pool and a potential difference between the pool and the ground.

This could be mitigated by ensuring the rubber matting you have provided at least a 1.5m overlap so that nobody could be touching true earth and your bonded pool.

2. Users inside the pool completely and equipment possibly bringing the water to mains potential.

This can be mitigated by using Class 2 equipment only or SELV only. Preferable the latter. An RCD plug on all your equipment also which would need to be tested periodically to ensure it works. I would not like to rely on the customers RCD protection not knowing the state of it.

Another option would be to provide an earth free equipotential bonding zone within the pool. This would mean all exposed conductive parts and extrenuous conductive parts would need to be bonded together so that they are at the same potential. The risk here is to people that are not in the pool and could touch the metal frame and true earth.

3. Users in the pool and able to touch electrical item outside of the pool that form a part of the building electrics.

This is an issue as you cannot really ask the customer to move their lights etc. So this would have to be surveyed and if any electrical equipment fell inside of the zones then you would have to decline that position of siting your pool.

As you can see there are so many things to consider each site will need to be thoroughly surveyed each time.

To be honest I would be inclined to suggest the following:

Pool only to be sited on insulated matting with a 1.5m overlap and so that no electrical iteams are within zones as per BS7671.

All electrical equipment to be Class 2 or SELV and be RCD protected by your own RCD plugs.

When people are in the pool all electrical items to be disconnected from the electrical supply and reinstated when all people are clear. Completely remove the risk when people are in there.

We always have a minimum 2m extension of the rubber matting all round the tank, so that's already in place. we also move all electrical outside of this area.

Everything on the supply side and our own equipment, and any other equipment attached anywhere in the space is on RCD and is tested as it is bought together and connections made.

I can and do keep to elv/selv within the 2m zone as the only stuff in there is generally my own design of SFX, which is all 12/24v. Beyond that 2m area though, none of the kit that you would find in a studio exists as class 2 or SELV.

In your scenario you're suggesting not earthing anything on or immediately around the tank then? That feels a little like, as I think you inferred yourself, taking one danger away from those in the tank and gifting it to those around the tank where the zone ends.

 

[DPG]

I know you said you can't drop to 110V, but are you able to feed everything in the area via a 10mA RCD?