Shunt Trip - Not Working Correctly

[ryanm]

Hi,

On a compressor at work, it needed to be E-Stopped, which should trip out the main Switch Disconnector...only it didn't.

The E-stop is on a 24v circuit, which only goes to a shunt trip, in the main Switch. So 24v -> E-Stop -> Shunt trip -> 0V.

Upon investigation, it seems if you press the E-Stop, release it, then activate it again, this will then activate the shunt trip! I tested this many times, and EVERY time it activated on the 2nd attempt!

Any ideas why this would be happening? Or how to fix it?

Thanks a lot

 

[ProDave]

You need to fundamentally re design the estop circuit.

If I am reading it right, the estop works by ENERGISING a trip coil to release the contactor.

That is NOT fail safe.

It should be reconfigured so the estop is a normally closed switch amd pressing it opens the contact thus de energising the contactor.

Perhaps you need to do a proper risk assesment and design a proper estop to EN60204, but possibly not needed if all the mechanical parts are guarded so the risk is low.

If you want to do further fault diagnosis on the existing circuit, you need to measure the voltage on the trip coil and see if the 24V is actually getting to the coil. If not, the switch may be dodgy. If the 24V is there, but it's not releasing on the first try, there's probably a mechanical problem with the release mechanism.

 

[ryanm]

Sorry, no the Shunt Trip is energised all the time, and when the E-Stop is pressed, the Shunt Trip de-energises. (apologise I didn't explain it correctly, but you are right )

The E-Stop is OK, I've had a look at this, as it is easily accessible/safe while the plant is running, but I've not had a look at the Shunt Trip part due to not being able to isolate the power until the weekend.

I'm thinking it is more mechanical if I'm honest. What things would normally go wrong with it though?

Thanks for the input.

 

[Sidewinder]

This is not correct, release of an e-stop must NOT re-start equipment.

 

[ryanm]

It doesn't re-start the equipment.

On the first attempt of pressing the e-stop, the shunt trip stays energised. Every 2nd attempt drops the shunt trip out, then the main switch-disconnector has to be reset.

I was wondering if anyone had any ideas why it would NEVER drop the shut trip on the first attempt, and ALWAYS drop it on the 2nd attempt. I'm thinking something mechanical, but what could go wrong?

Thanks

 

[ProDave]

Have you yet actually measured the voltage across the shunt trip? Knowing if that changes on the first press of the estop is crucial to determining the fault.

Does the "normal" stop button stop the compressor on the first press, or does that need two presses as well?

 

[ryanm]

Hi Dave,

No I was going to try to get in to it this weekend, but they were using it. I will try to plan it in at the beginning of the week though (depends on the production schedule).

The 'normal' stop button does stop the motor on the compressor yes, but there is still power on the panel etc. This isn't a normal stop button as such though as the compressor ramps down first so no damage is caused. So you have to stand there a short while pressing the button, which isn't too save in terms of an emergency.

The E-Stop de-activates the shunt trip, which in turn trips the main switch (Schneider NSX 630A), so the whole compressor and panel has no power to it.

Dave, what is the logic to see if the voltage changes on the first press, then de-activating it on the 2nd? I suppose you might mean if it does change from 24v to 0v that everything electrically is working, so it has to be mechanical? Or am I barking up the wrong tree?

Thank you for your time and input!

 

[ProDave]

Dave, what is the logic to see if the voltage changes on the first press, then de-activating it on the 2nd? I suppose you might mean if it does change from 24v to 0v that everything electrically is working, so it has to be mechanical? Or am I barking up the wrong tree?

Thank you for your time and input!

Yes precicely

If pressing the stop button removes the 24V first and every time, then the failure to release is a mechanical problem in the shunt trip mechanism.

If the 24V remains, then the contacts of the estop button are stuck and that needs replacing. (a proper estop button shouldn't do that as the contacts must be forced open by the switch and must not rely on a spring to open the contacts)

 

[Sidewinder]

What compressor is this please?

Is it an air compressor, if so what type, what is its function.

What motor is fitted?

What is the drive between the motor & compressor?

How many e-stop buttons are there?

What is the physical layout of the control system please?

Yes there is method in these questions.

 

[Sidewinder]

I was being a "BIT" more tactful!

Hence the barrage of questions...

Anyway, I agree with the Boat Paddler above.

However, we NEED more information to give suitable advice.

 

[ryanm]

What compressor is this please?

It's a Sabroe refrigerant compressor

Is it an air compressor, if so what type, what is its function.

What motor is fitted?

A Sabro supplied one, controlled via an inverter

What is the drive between the motor & compressor?

The motor is directly coupled to the compressor

How many e-stop buttons are there?

1

What is the physical layout of the control system please?

There is a small control panel on the side of the compressor, and a large panel at the end, which contains the inverter, contactors etc.

Canoeboy, the system is:

e-stop -> shunt trip, which in turn trips the 630A switch. What is the issue with that? Schneider designed it to work like that?!

This is similar to what I am on about:

 

[ProDave]

Have you done any measurements yet as I asked a few posts back?

 

[Sidewinder]

I very much doubt that Schneider designed the control system!

I suspect that the OP means that Schenider have designed, made and put onto the market equipment which when connected together functions in this way.

However, I doubt that the application of this "shunt-trip" & MCCB are in accordance with that intended and envisaged by Schneider!

This method of shutting down does seem unusual and is not in general compliance with the New Approach Directives, or the associated standards.

What is the refrigerant?

Is this in an ATEX zone?

 

[ryanm]

Dave, sorry yes I have tested it, and it drop to 0v every time the E-Stop is pressed. I've phoned Schneider for advice, and they said it's not actually a shunt trip as it is always energised, so is actually an Under Volt Release. To be honest they've never heard of anything happening like this before, so recommended replacing the under volt as it is relatively cheap to try. So I've ordered one so we will see if this solves the issue.

It is an Ammonia screw compressor, correct. It is approx. 10-12 years old.

Canoeboy/Sidewinder, can you explain the issue to me please with the way it has been designed? I don't know what the issue is, and why I use this site to gain more information and learn. Obviously you guys can see something very wrong here, which is fair enough as I'm not doubting you, but what are the issues?

The reason this issue came up was a bearing failed in the motor driving the compressor, and due to thisthe rotor was actually spinning on the stator, which caused a lot of sparks and smoke! The E-Stop was pressed which should have shut the whole system down....but didn't! So needs sorting obviously.

Thanks a lot

 

[Sidewinder]

OK,

Just quickly as I have popped onto the forum via my mobile while on site.

The first issue has been explained to you by Schneider.

So this should be corrected first, the shunt trip should be used to detect mains failure, and trip the breaker for safety reasons if the supply fails and it is deemed necessary.

Will be back later.

 

[ryanm]

This is what it does do now, if mains fails, or the E-Stop is pressed, the switch trips.

 

[Sidewinder]

OK so one of the functions may be correct in that a concious effort has to be made to re-start the machine / re-energise after mains failure.

Are there any safety related systems on the compressor which rely on power for a safe and controlled stop?

 

[ryanm]

In terms of an emergency stop, no, nothing relies on power to stop the compressor.

When it automatically stops, or if you were to manually stop it, yes it needs power in terms of the display, sensor readings etc.

Thanks

 

[Sidewinder]

What I mean by my query is is there for example a separate lube pump for the unit which would continue to be powered under a controlled stop?

Also more importantly are there any safety related devices connected to the unit which rely on power to continue to operate safely in the event of a shutdown?

 

[ryanm]

Hi,

Yes there is a lube pump, which continues after the compressor has stopped. Although this does stop too if the e-stop is pressed.

No, there are no safety related devices which rely on power.

Thanks