Latching Circuit

[ryanm]

Hi all

I am hoping you guys could help me out with something. I'm wanting to add a small circuit to an existing panel at work, as recently there was a leak from a hot oil pump, and it had the potential to catch fire (there was lots & lots of smoke).

So I'd like to fit a smoke detector so everything will stop, which is simple enough to do, but I don't know how to make it latch? As if there was some smoke, it would activate a relay stopping everything, but once the smoke cleared the system would start up again. I'd rather it latch on even if the smoke does clear, and someone would then manually have to press a reset button.

This is where I'm stuck as I don't do a lot of things like this, so I'm hoping you guys could point me in the right direction? I see on RS Components you can get latching relays, which sound perfect, but can these be reset with a simple N/O push switch?

Buy General Purpose Relay DPCO latching relay 10A 230Vac coil Releco C3-R20 / AC 230 V online from RS for next day delivery.

This is the simple circuit which will work ok, but won't latch in the case of the smoke clearing:

http://i570.photobucket.com/albums/ss141/mcarthyryan/LatchingCircuit.jpg

Any pointers would be great.

Thanks

Ryan

 

[Sidewinder]

ryan,

Is the machine CE marked?

If so then you will need to re-CE mark the machine after your mod to comply with the law.

Else I would put the contact of the smoke in the e-stop chain of the machine.

Does this make sense?

---------- Post Auto-Merged at 19:04 ---------- Previous post was made at 19:02 ----------

Just looked at your drawing again, is the machine using 230V a.c. control direct from the mains?

 

[Phoenix]

You probably dont need a latching relay...

Does the machine have a stop button already? use the relay in the smoke base in series with that so its effectivly another stop button.

 

[Andy™]

you will need a contactor wired as push to start and stop. but the project may have other implications & requirements as above

the circuit you need is quite basic. if you have to ask, you probably shouldnt be trying it

 

[ProDave]

It's not a Latching circuit for the smoke detector that you need.

What's needed is a contactor that's always energised (started with a "start" button or similar)

The contactors main contacts switch the load (oil pump) and an auxilliary "hold on" contact keeps the start button circuit bypassed to keep the relay energised.

The smoke alarm would ideally have a normally closed contact, that opens when smoke is detected. You won't get that from a domestic smoke alarm, so look for something more professional.

That normally closed contact is wired in series with the contactor coil, so when the alarm goes off, the contactor drops out and stops the pump.

Of course that circuit described just illustrates the function of the circuit needed. It's most unlikely that simple circuit would actually be adequate.

Now you need to do your risk assessment according to BS EN 60204 to see what category of circuit you need. It may demand redundancy, two contactors, a safety relay with contact monitoring and a dual circuit stop function.

If that last sentence has baffled you, then I'm afraid you are not qualified to make this modification. I used to be 8 years ago, but because I have not kept up to date with changes to 60204, I am almost certainly not qualified to design such a circuit now.

 

[Sidewinder]

Dave,

They have still not decided on the replacement of 951 properly mind!

 

[mikep]

Building block for latching circuit using relays.

You can make a relay latch by simply using one of the sets of contacts to supply a live feed back to itself onto the switched live this will make it latch on, for the reset simply put a normally closed pushbutton switch in the neutral feed, pressing this will break the neutral feed opening the relay to reset as long as there is no trigger feed.

So when it gets a live trigger feed it closes contacts then supplying it's own supply to keep it latched then to break it press the reset button to remove the neutral momentarily.

That part is for the latch, use another set of contacts on the same relay to control the motor feed.

Regards, Mike.

 

[Sidewinder]

mike,

It's now illegal to use machines that have just been modified without sufficient "other" considerations, so unfortunately you can't just mod a machine these days!

Just like you are not allowed to use a phase and neutral or 2 phases for control on anything but the absolutely simplest of machines.

 

[mikep]

I take your point on that Sidewinder and agree, others on this thread have already given their opinions and thoughts on this but no one has mentioned how to achieve it.

He has already been told of the implications by other members of problems arising from such a circuit on the legal side and thoughts of design.

However I was just explaining the principle of latching relays for Ryan and others to understand, I have not gone through any process of circuit layout but have just shown an explanation of how a simple latching relay works, that is why I put it down as "Building block" and no real detail for a design to use.

Regards, Mike.

 

[Andy™]

I take your point on that Sidewinder and agree, others on this thread have already given their opinions and thoughts on this but no one has mentioned how to achieve it.

i did, in post 4

and prodave said same thing but more detail.

and there are some off the shelf items that would do the task with a little modification. no volt trip switch. or a DOL starter

 

[ryanm]

Thanks a lot for all your replies.

I'll be honest, that a lot of things I've learn't are through books due to my education/apprenticeship not being the best if I'm honest, so will openly admit to anyone that I have a long way to go.

Andy, I appreciate you saying if I don't know the basic circuit, I shouldn't be trying it. But I need to learn things, and forums like these are great for learning. Otherwise I will end up doing things wrong, as I'm sure you can appreciate when employed by someone you sometimes need to get the job done. As I'm new in to my career I can't tell my employers where to go yet I don't mean that paragraph to come across cocky, as it is mean't with respect.

Sidewinder, I don't know if the machine is CE Marked or not really. There are no markings on there other than the manufactuers name plate. Also, yes the control is 230Vac, using one phase of the 3 phase for the motors etc. I was going to use 24Vac for any modifications I was going to do with the use of a tansformer.

Canoeboy, I haven't yet found said smoke detector yet, but was going to look for something from RS components, rather than a domestic market.

ProDave, again I will be honest. I don't know much about BS EN 60204, and haven't been taught anything about it at college.

mikepitts, really appreciate the explaination of how it works! I will draw it out tomorrow as drawings give me a better understanding of things generally.

Other than buying Safety of machinery. Electrical equipment of machines. General requirements BS EN 60204, is there any courses I could go on to increase my knowledge more of this subject?

Thanks a lot again

---------- Post Auto-Merged at 01:16 ---------- Previous post was made at 01:06 ----------

From a quick google, I think City & Guilds 1623 would be beneficial, and hopefully I should gain the knowledge needed for the above query?

 

[Sidewinder]

OK, ryanm,

If the machine was made after "around" 1994 then it should be CE marked, else it was not sold in compliance with the law that has existed since then.

If it is older than that then it it will not be CE marked.

You should have a copy of any relevant standards really to the work that you are doing, so there is 60204-1, 951, 1050, 12100 and a load of others.

I also normally start with pen and paper when designing a circuit mod the old a picture is worth a thousand words adage.

You would not normally cover 60204 at college as it does not form part of the syllabus for the 2330, 2391 etc or related courses as it is not a standard for the electrical installations in buildings.

You need to select your detector carefully to ensure that it is fit for purpose really. RS may be an option, though it may not I have not looked.

You need to be careful with your selection else you may be just providing a false sense of security.

Perhaps there is another better solution for the issue, without knowing the machine, process and failure mode it is difficult to say whether the smoke detector option, or investigation and addressing the root cause would be better.

If you do go the smoke detector route, then I would recommend one that has a normally closed contact which opens on detection, if this is a suitable safety integrity level.

This would need to be assessed in conjunction with the whole machine control stop chain.

If the machine is using 1ph & N as its control then that does not comply with current law, and it is considered retrospective when you undertake modifications in my opinion and that of many others.

You must supply the control from a transformer with suitable circuit protection on the primary and secondary and it must be below 266V a.c. (from memory, it is a really odd value!).

Another issue you have is that you are undertaking this modification to increase the safety of the machine, thus it becomes a safety related part of a control system, this brings in a whole new raft of standards and checks.

As ProDave suggested, is a single channel stop chain acceptable, would you need safety relays, is the rest of the circuit up to the required SIL?

One of the problems (read good things) about machinery is that it is covered by statute law!

It makes it a nightmare to work on if you don't know, but, it should mean that things are done correctly, the trouble is most don't know the law!

HTH

 

[dannyr]

You could have used an ABB across the line contactor.

 

[Andy™]

You could have used an ABB across the line contactor.

last post almost 2 years ago...

 

[dannyr]

Sorry. I didn't realize that I was reviving an old thread.

 

[dannyr]

You could have used an ABB across the line contactor.

Added a link to the ABB contactors if anybody's interested.