Protection and earthing of 24v ac circuit to energise solenoid valves

[Deuce]

Hi,

I am tasked with switching some special FX kit we have over to 24v ac, from 12v dc. Purely to achieve longer cable runs with lesser volt drop.

We have these transformers: link

Which are bonded to earth and powered via a MCB and RCD (we work with water) protected feed, as with all our kit.

But in the assembly I currently have, there is no protection or bonding on the 24v side. Should there be? What is the regulation? It's a double edged sword for us as if I add a 24v ac breaker, or simple fuse, I run the risk of inrush current tripping/blowing. Not ideal for special FX designed to operate repeatedly on cue! And the control box could one day be running 2 solenoids, the next day 20.. So matching protection to the current demand is problematic also. Should we instead use a over-current protected power supply - or will that also potentially break the circuit due to switching multiple solenoids?

And should the earth from the 24v solenoids be bonded to the mains earth?

Thanks in advance for any guidance.

 

[The Ghost]

Well what strikes me is any attempt to bond to earth from the 24v side to the 240v side would be unwise. If you think about it a fault on the 240v side earth could travel through to the 24v side causing abnormally high voltages. Again with water involved (although I can't quite visualise what your are meaning) Notionally anything above 50v a.c. is considered dangerous. Therefore I would suggest you would be aiming for a situation where you have SELV so that the earth is separated from the ELV side. Ah just looked at your link for the transformer which is an isolating transformer, so you don't want to undo the "isolating" safety feature by connecting the earth on to the mains from the ELV side.

 

[westward10]

If there is water involved I would use 12v SELV.

 

[Deuce]

If there is water involved I would use 12v SELV.

Thanks but there are two requirements to keep in mind.

1) It has to be safe, within regs.
2) It has to perform the intended function.

There are big problems switching several solenoids in parallel using 12v due to the inrush current and the lengths of cable. 12v limits the distance and number of valves we can switch.

For the same reasons, irrigation systems commonly use 24v ac. That is effectively what we're re-creating here, albeit for a different purpose.

We use 2.5mm cable. Typically 20m from the power supply to the first solenoid. Another 20m to connect to all solenoids, then 20m back to the power supply. So we create a 60m 2.5mm ring circuit back to power supply, with all the valves branched off from that. I think with 12v power supply pushed up to 13.5v, we still only saw 8v on the far side of the circuit. That leads to staggered and intermittent operation of the valves.

 

[Deuce]

Well what strikes me is any attempt to bond to earth from the 24v side to the 240v side would be unwise. If you think about it a fault on the 240v side earth could travel through to the 24v side causing abnormally high voltages. Again with water involved (although I can't quite visualise what your are meaning) Notionally anything above 50v a.c. is considered dangerous. Therefore I would suggest you would be aiming for a situation where you have SELV so that the earth is separated from the ELV side. Ah just looked at your link for the transformer which is an isolating transformer, so you don't want to undo the "isolating" safety feature by connecting the earth on to the mains from the ELV side.

Thanks, I had assumed that it would be odd to bond primary and secondary earth. So, no earth on the 24v side then? Looking at the valves, some have earth pins, some don't. I assume the ones that do are because the are brass, and the same body is used for 240v versions.

As it is ELV, is there any requirement to bond at all?

I'm only concerned with not being criticised really. The water element is the reason we use lower voltages, as it essentially removes the risk. 24v can in theory be dangerous. But 24v also passed through water... water is actually rubbish at carrying current.

Still leaves a question mark over circuit protection though. 24v can still get hot and start a fire so I feel there should be something in place just in case for whatever reason, it were ever to get that hot. Can't think how/why it should but..

 

[Darkwood]

Can you draw a sketch diagram it's hard to see exactly how all this works and when you say 'to regulations' you do realise this is out of the scope of the BS7671, your treading into the realms of machinery control etc even though your just switching solenoids - the requirements to design will be similar, the earthing arrangements are flexible on extra low voltage and denoted on how you arrange your fusing, strapping one leg down of the 24v will change the acceptable fusing arrangement that is permitted, you should be looking into the BS60402-1 for guidance on that, it will also show you the colour coding requirements of control wiring and that of power circuits.

 

[Darkwood]

Well what strikes me is any attempt to bond to earth from the 24v side to the 240v side would be unwise. If you think about it a fault on the 240v side earth could travel through to the 24v side causing abnormally high voltages.

Most transformer chassis are earthed through, it is common in control equipment to strap down one side of the output to common earth, this poses no issues even of that you have mentioned, I often strap down on the 24v side as this allows for simpler and easier fault finding if you can reference the control voltage to any local earth, don't mix this up with an Isolating transformer like found in a shaver outlet where the output has no earth reference thus safe to use in a bathroom even if it went faulty.

 

[Deuce]

Can you draw a sketch diagram it's hard to see exactly how all this works and when you say 'to regulations' you do realise this is out of the scope of the BS7671, your treading into the realms of machinery control etc even though your just switching solenoids - the requirements to design will be similar, the earthing arrangements are flexible on extra low voltage and denoted on how you arrange your fusing, strapping one leg down of the 24v will change the acceptable fusing arrangement that is permitted, you should be looking into the BS60402-1 for guidance on that, it will also show you the colour coding requirements of control wiring and that of power circuits.

Sorry on the move - got this project with stupidly short lead time so rushing around.

It's very simple though. Mains to transformer. Then the 24ac output is sent around a ring circuit of 25mm cable, with each solenoid taking poor of the ring (wired in parallel). At present there is no bonding on the 24v side at all. It could in theory make a structure/person live if damaged and exposed - but of course, only 24v so probably no one would notice unless they were investigating using their tongue...

The other aspect is that the valves for the most part don't even have an earth wire/terminal. See first few seconds of this video for a similar valve type. No earth..

If there was nothing specific saying it should be bonded, I would be happy to simply not bother. They will be mounted away from people on a aluminium truss frame, which is itself grounded, like all steel/alloy structures.

 

[Darkwood]

Just to add here, there are other issues that must be understood and accounted for when designing control systems and that is floating voltages, example - you can have a DC speed drive card that puts out a 0-10v reference to a pot' so the user can control the speed of a motor, some drive cards do not have galvanic isolation and thus a floating voltage can appear on the 0-10v, this will not interfere with how the drive works but can be hazardous to the user especially if the pot becomes damaged or gets wet, they can receive nasty belts or worse from what is a 0-10v signal device.. I have seen voltages on these systems as high as 230v plus so heed caution, having said that if all your doing is creating an extra low voltage to operated solenoids then this shouldn't be much of a concern.

 

[Darkwood]

Sorry on the move - got this project with stupidly short lead time so rushing around.

It's very simple though. Mains to transformer. Then the 24ac output is sent around a ring circuit of 25mm cable, with each solenoid taking poor of the ring (wired in parallel). At present there is no bonding on the 24v side at all. It could in theory make a structure/person live if damaged and exposed - but of course, only 24v so probably no one would notice unless they were investigating using their tongue...

The other aspect is that the valves for the most part don't even have an earth wire/terminal. See first few seconds of this video for a similar valve type. No earth..

If there was nothing specific saying it should be bonded, I would be happy to simply not bother. They will be mounted away from people on a aluminium truss frame, which is itself grounded, like all steel/alloy structures.

If you think about it, if a fault occurred on the 24v side and one went down to exposed metal work then if you have not strapped down either leg then the person would have to be in contact with both the metal work and the other un-grounded leg to get a PD of 24v which is not going to give them any real noticeable shock, strapping one leg down gives a few advantages in that a damaged cable would have a higher chance to blow a fuse and faulting the circuit can be done more quickly although the latter can still be done by simply grounding one leg for testing purposes only.
The call is yours and should be based on the type of control system, the equipment been used, the risk of problems occurring be it faulty equipment or a higher risk of damage/wear and tear etc and the ability for the circuit to respond accordingly to maintain safety.

I cannot tell you what is the best solution for your particular circumstances as it varies widely but which ever method you choose you need to understand what fusing solutions that go which it.

 

[Darkwood]

Just an additional note here, the special effects industry is heavily regulated and as I have no insight to what you are actually doing then all of which I have explained may not be applicable to your situe', I would seek the relevant regulations to cover this sector and if there is explosives etc used then there will definitely be some very strict guidelines and standards to follow even in the control side.. you can imagine you do not want a device going off by accident because of a bad control design.

 

[Deuce]

Just an additional note here, the special effects industry is heavily regulated and as I have no insight to what you are actually doing then all of which I have explained may not be applicable to your situe', I would seek the relevant regulations to cover this sector and if there is explosives etc used then there will definitely be some very strict guidelines and standards to follow even in the control side.. you can imagine you do not want a device going off by accident because of a bad control design.

Like any industry, all the common parts are heavily regulated. So explosives, yes of course. But we deal solely in water FX which is so niche, there is nothing specific. Other than obviously basic electrical good practice and water contamination guidelines etc.

 

[Darkwood]

Like any industry, all the common parts are heavily regulated. So explosives, yes of course. But we deal solely in water FX which is so niche, there is nothing specific. Other than obviously basic electrical good practice and water contamination guidelines etc.

Fair do's ... yes you did mention it earlier but was unsure how far you went with effects, even water systems will have heavy regulation if they are pressurised or pressuried gasses are used, any system that component failure or control failure could cause serious injury or death, if there is little pressure used then possible not an issue.

 

[The Ghost]

@darkwood you say it is not to be confused with shaver transformer (isolating type) but it does say on the transformer link it is EN 61558-2-6 which I understand is isolating tranformers, and or SMPS types as well. Slightly lost here on BS EN standards. As I can't afford to buy them all let alone the time to read and digest.

 

[westward10]

This is totally off the wall and related in a tenuous way. Back in the eighties I was involved with SFX and a couple of old boys I worked with showed me how they recreated machine gun tracks across the floor in war pictures back in the fifties. We were down the Lot at Pinewood and they laid some parallel charges beneath some leaf cover with each one wired back to a row of nails on a plank of wood, a wire was taken from a car battery and with the bare end it was flashed along the nails creating a machine gun track, brilliant