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

[Darkwood]

@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.

What I meant was not to confuse it with how a shaver outlet is connected on its secondary side, yes TX's in control equipment are often isolating as is the one he has chosen but how you arrange the earthing is down to the designer, Isolating TX's can be strapped down on the secondary if required, output voltage is often key to whether this is a safe method and as 12 or 24v is extra low voltage then it is the choice of the designer.. if you think about it just take a typical onsite 110v TX and look how the earthing is done.

 

[Deuce]

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.

Well this is where I am with my thinking too. There is no way of anyone accessing the valves/connection points themselves, so it's basically just metalwork the system is attached to that could transfer current to within reach or a person. And that metalwork is earthed to ground in anycase. Fault finding is also straight forward there is a twist lock connector every 8m cable of the ring circuit so it can just be broken into smaller parts and tested that way. Would only take minutes.

I am indeed most worried about circuit protection. What would you use on the 24v circuit? It needs to be something that won't trip/blow due to the inrush current when the solenoids are first energised (could be 20 at once). The number of valves and therefore the load varies though. But obviously won't exceed the capacity of the cables.

Is there an MCB that would be suitable for 24vac use? We have only used fuses previously (the power supplies were fused). These 24vac are not. Would prefer MCB to fuse as it's faster and instant reset.

 

[Darkwood]

This is where you pass the boundaries of needing helpful advise and are really requiring a design doing , I have little info to go on as I have no ratings for valves or peak inrush nor do I have frequency or duration or usage all of which can effect the requirements of the front end TX, I also don't want to get drawn into actually designing it for you, it would be an idea to set up a test bench and design it to the worse case scenario and get a TX oversized for your needs, although you will have to consider on sight power supplies too, you don't want to be tripping them when you plug in the main TX.
There is a lot to consider when you design such a project of which some I haven't even touched on, if the solenoids are 12v DC then you have to consider high voltage spikes as the solenoid field collapses when switch off, this can do a lot of damage to control equipment if you don't put in some form of flyback diode.

 

[Deuce]

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

Oh the same kind of shenanigans still happen in the SFX world

Generally even when something is lashed up in a hurry and not perhaps 'ideal' common sense prevents actual danger. I'd say pretty much everyone I know in the industry is very capable of making things safe, regardless of whether or not a chap holding a clipboard would agree it is within regulation.. I could give some humorous examples but best not to tell tales.. I will go as far as to say if it starts to rain outdoors, a lot of non weatherproof equipment is instantly upgraded to IP68 with the use of bin liners etc.

 

[Deuce]

This is where you pass the boundaries of needing helpful advise and are really requiring a design doing , I have little info to go on as I have no ratings for valves or peak inrush nor do I have frequency or duration or usage all of which can effect the requirements of the front end TX, I also don't want to get drawn into actually designing it for you, it would be an idea to set up a test bench and design it to the worse case scenario and get a TX oversized for your needs, although you will have to consider on sight power supplies too, you don't want to be tripping them when you plug in the main TX.
There is a lot to consider when you design such a project of which some I haven't even touched on, if the solenoids are 12v DC then you have to consider high voltage spikes as the solenoid field collapses when switch off, this can do a lot of damage to control equipment if you don't put in some form of flyback diode.

Our kit is pretty basic really. We use a PLC to handle control, which is EMF sensitive. But we then use the on board relays to switch over larger relays. So nothing feeds back to anything sensitive. The PLC is on an independent circuit altogether.

We use relays with flyback diodes too. Never had any issues. I have heard it's a far greater issue with higher voltage solenoids, or other inductive loads. I assume because the lower voltage coils can only build a flyback charge that is capped below the level at which it can do much if any harm. It's tricky for us to do these things 'perfectly' as the reality is the kit is all modular and gets hooked up in different configurations all the time. We take basic precautions to make it as reliable as possible, and certainly to make it safe.

As for circuit protection - seems I could just buy a load of these at different ratings and swap over to the rating most closely matched to the demand on the circuit each time (without exceeding cable limits). We do testbed the systems each time so no issue to fire the solenoids on and off a hundred times to make sure they don't trip it. In addition to testing the operation of the breaker within the circuit.

https://uk.rs-online.com/web/p/mcbs/7913290/

 

[Deuce]

Sorry here is the link I meant to post. These are available type c and from 2-16a.

https://uk.rs-online.com/web/p/mcbs/7913290/

So all I have to do is calculate the maximum in-rush to choose the suitable MCB each time.

 

[Darkwood]

You gave a link that shows a 100w TX 400/230 - 24ac

If you look at the product image it does tell you that the output fuse will need to be 4amp gG, this may blow with multiple solenoids so a 4amp gM may be wiser, if this still has issues then you need to consider a larger TX, if you fuse the secondary side correctly then the primary can be fused as short circuit protection only and you can uprate it to suit the inrush requirements, I would strap down your secondary on one half and fuse the other side as suggested on the TX - ohms law gives us that rating fuse of 4amps.

 

[DaveHsussex]

Well this is where I am with my thinking too. There is no way of anyone accessing the valves/connection points themselves, so it's basically just metalwork the system is attached to that could transfer current to within reach or a person. And that metalwork is earthed to ground in anycase. Fault finding is also straight forward there is a twist lock connector every 8m cable of the ring circuit so it can just be broken into smaller parts and tested that way. Would only take minutes.

I am indeed most worried about circuit protection. What would you use on the 24v circuit? It needs to be something that won't trip/blow due to the inrush current when the solenoids are first energised (could be 20 at once). The number of valves and therefore the load varies though. But obviously won't exceed the capacity of the cables.

Is there an MCB that would be suitable for 24vac use? We have only used fuses previously (the power supplies were fused). These 24vac are not. Would prefer MCB to fuse as it's faster and instant reset.

Can't you just fuse each valve? I know it's more work, but it would scale with capacity that way. If you had a dead short in a single valve, it would blow the fuse locally at the valve, rendering it dead, but keeping everything else alive and working for your FX.

If there is a dead short in the cables, well...you gotta ask yourself what over current can your transformer take on the secondary side before it dies, is that 4amp on the label? You would have to fuse low anyway to protect it, and fuse the solenoids even lower to at like 1amp so a failure can't pop the main fuse...

 

[PEG]

Hi bud,just three questions...

What is the holding current or rating on those valves?

Can you not use a pilot operated valve,to reduce current?

When is Waterworld II being released?

 

[Lucien Nunes]

Agreed, the transformer is what needs protecting here, not the cable. The 100VA tranny with its 4A maximum output would be toast before any damage was done even to one leg of 2.5mm² let alone a ring. The valve wiring and coils are a different matter, maybe there is value in putting small fuses in the tap-off points.

Ideally you should fuse to fully protect the transformer against continuous overload (i.e. it should eventually trip before the winding exceeds a safe temp) but as it has high thermal mass the protection can be relatively slow. A thermal CBE (Circuit Breaker for Equipment) might suit - you don't need magnetic protection as the short-circuit current is small compared to a mains circuit and might not even be high enough to reliably trigger a magnetic trip if the fault is at the end of a very long run. Not having the instant magnetic reponse will help avoid nuisance tripping on inrush.

Have a look at something like http://www.e-t-a.co.uk/fileadmin/us...rungsautomaten/Thermisch/2_eng/D_1658_ENG.pdf

 

[Deuce]

Can't you just fuse each valve? I know it's more work, but it would scale with capacity that way. If you had a dead short in a single valve, it would blow the fuse locally at the valve, rendering it dead, but keeping everything else alive and working for your FX.

If there is a dead short in the cables, well...you gotta ask yourself what over current can your transformer take on the secondary side before it dies, is that 4amp on the label? You would have to fuse low anyway to protect it, and fuse the solenoids even lower to at like 1amp so a failure can't pop the main fuse...

Thanks,

The issue then is if a fuse blows, it's 8m up in the air above a giant pool of water. So best to keep protection at source of the circuit.

 

[Deuce]

Hi bud,just three questions...

What is the holding current or rating on those valves?

Can you not use a pilot operated valve,to reduce current?

When is Waterworld II being released?

17va holding. And there will be 9 on the circuit. The problem with pilot op valves is that they are prone to failing to close of even a tiny bit of debris finds it's way through. The solution is to use a direct acting valve, as we do. But yes, direct acting valves do have up to 10x the current draw.

As for waterworld 2... I think it took so long to break even the first time it's one film there may not be a sequel of But it's an excellent example of how expensive doing anything with water is. Many films and TV shows end up cutting intended features when they see the cost of making water tanks/fx safe. Although it's also true that sometimes 100k will get spent on something that never actually makes the final edit.. I love my job - sometimes.