Safety circuit contactors issue

Hi all,

I have an issue with a machine safety circuit.

The circuit contains a safety relay which when deemed in a safe condition can energizes two redundant contactors if both contactors are in the same state. The contactors have several normally open contacts as well as one each normally closed AUXILARY contact.

There is an electrical path to energize both contactors if both contactors are off and so a path through both contactors normally closed auxiliary contacts.

If both contactors are then on, there is a path through both contactor normally open contacts to remain on. (Safety relay also permitting a common path to +24VDC).

The problem arose when I had to replace the two contactor, which DID have all the required NO / NC contacts however both contactor do NOT have auxiliaries for the two NC contacts, as they were part of the main contactor.

Both contactors now just chatter on and off.

In testing other (unusable in this application) contactors with auxiliaries, I found that the normally closed contacts do not actually open UNTIL the normally open contacts are already closed! Both are actually close for a brief “pull in time”.

My questions are this:

Is this by design and how the original circuit might have worked?

Is there a natural delay between a contactors NC contacts opening until the NO contactors closing to ensure a path in both states (again unless the safety relay has already broken the path)?

Do all auxiliary contacts perform this way in relation to the main contacts by design for just such a requirement?

I do know if I order replacement contactors with NC auxiliaries the issue will be corrected for a few months until the contacts go bad (we have done this three times already). The contactors we try to add WITHOUT NC auxiliaries (but all the correct NO / NC main contacts will not work (just the chatter).

I would appreciate thoughts and info on this subject.

It is possible to get late-make/early-break or late-break/early-make contacts. Possible, but as I found when looking for just such an item, extremely difficult, especially in safety contactors. But from what you're describing, it sounds like this could have been what was originally there.

Can you post a schematic diagram of the circuit? It might help us understand what's going on a little better and you might get some more insightful responses

the original contactors have what is called "Positively Guided" contacts
this means they are fixed permanently and forced ope or closed by the main contacts.
you can not use a standard set of contactors

your fault is because they are not operating in the correct timing sequence

Not sure if these would be proper to use in a safety circuit or not but most manufacturers sell aux contact blocks with time delays.

You could use the N/C contact on the Aux Time Delay block and the N/O contact on the Contactor itself.
This should function the way you are looking for, N/C would be held for a fraction of a second allowing time for N/O contact to to completely close tight.

Here is an example of one from Schneider.

http://www.alliedelec.com/schneider-electric-ladt2/70007846/

BCS

Description sounds like a makeshift safety relay with monitoring, it would be wiser, and probably safer, to use a safety relay made specifically for that purpose. Pilz PNOZ series is a popular choice.

It is better to purchase additional safety relays. Like Omron G7 series.
Wire them in series with the N/C check area.
Most Aux relay contacts are not positivley guided.
Although if you only want indication than there is no problem.

Ian,

I fully concur with this assessment...

iant said:

the original contactors have what is called "Positively Guided" contacts
this means they are fixed permanently and forced ope or closed by the main contacts.
you can not use a standard set of contactors

your fault is because they are not operating in the correct timing sequence

Click to expand...

The timing is all wrong for a safety application because the contactors are not safety rated, using positively force guided contacts. The switching of the contacts is indeterminate.

The NC contacts are most likely wired to the safety relay's reset circuit for a monitored reset. If it's a manual reset, then there will be a push button also in series with the NC contacts. When the contactors are in a de-energized state, the NC contacts will provide a closed circuit, awaiting the reset push button to be actuated to reset the safety relay.

If the safety channels of the safety relay are healthy, and the reset push button actuated, then the safety relay's auxiliary contacts will close, energizing the contactors...

Corbis said:

...If both contactors are then on, there is a path through both contactor normally open contacts to remain on. (Safety relay also permitting a common path to +24VDC)...

Click to expand...

...This is usually a supply through the safety relay directly to the coils, and not via NO, or other auxiliaries on the contactors.

Once energized, the NC auxiliary contacts, on a safety rated contactor, will open before the NO contacts close.

It sounds to me like you may have extra and possibly unnecessary wiring through the contactors auxiliaries before going to the coils? Either this, or this and the timing of the contact switching are possibly causing your chattering?

As ASF requested, a wiring diagram would be most useful to us.

Attached is an example of an AB safety relay and how it would generally be wired to the safety contactors. Note how the contactor coils are wired directly from the safety relay's aux. outputs, and not via any other contacts. Also note the NC aux. contacts of the contactors and how they are wired to the reset circuit.

Disclaimer: This is just an example. I am not instructing you to wire your safety circuit as per the drawing. I do not know your machine and the safety function required from the risk assessment.

Remember, once you alter the existing safety system, you are now obliged to ensure it functions correctly. Chattering and frequent replacement of contactors is not just a nuisance issue here. It is most likely affecting the performance level of the necessary safety function.

Regards,
George

Safety Circuit Issue

Hi All,

As requested see attached PDF of the safety circuit schematic.

To clarify in case difficult to ready text, 24vdc(VE) is provided by the A21 Bircher relay if contacts 11(1) and 14(3) are made to both the K21 and K22 contactor coils, but only if one each normally closed contacts on both contactors are made or one each normally open contacts on both contactors are made.

The chattering on and off of both contactors (Seimens 3TH43) still exists and wouldn’t your expect them to do just that? When the two contactors try to pull in, the 24VDC will be lost from the NC contacts opening before the NO contacts close, and then repeat the cycle, thus the chatter.

A machine technician was at our plant this evening and he said he will have to check with the electrical engineers, as he also thinks this circuit does not make sense to work.

I would very much like to hear thoughts from those interested.

That makes more sense now that I've seen the drawing.

First, a disclaimer: I don't know how the Bircher relay decides to turn on the safety contactors, and for the purposes of your problem, it doesn't matter. I will assume that the operation of the Bircher relay is correct, and will output 24V when the safety circuit is healthy and the safety contactors should energise.

Now, with that out of the way:

You will notice two little "tails" on the contacts feeding into the contactor coils. The tails on the left (NC) contacts mean that these are "late break" contacts. The tails on the right (NO) contacts mean that these are "early make" contacts.

The purpose of that schematic would have been checking to ensure that both safety contactors came in and out together. Here's how it would have worked initially:

1. Bircher outputs 24VDC
2. Power flows through the two NC contacts, and energises both coils
3. As coils are energised, the contactor starts to pull in. The first contacts to close are the two early-make NO auxiliaries. Now the 24V from the Bircher can flow through both the NO and NC branches into the contactor coils
4. The main contactor poles close
5. The two late-break NC contacts open. Power continues to flow through the NO auxiliaries and holds them in

This would detect a welded-in contactor - as one of the NC auxiliaries would never close, the sequence would never get past step 1 above. It would also have detected one contactor not coming in when it should (or too slowly), because the NO auxiliary on that contactor would remain open, so once the late-break NC opened on the functional contactor, the circuit would be broken, and well, then you would see chattering much like you are seeing now.

So your problem now is going to be finding safety contactors with a NO early make and a NC late break auxiliary. As I mentioned in my first post, I've found them all but impossible to get my hands on. To be honest, if you can't source exact replicas of the original contactors, then your best bet is probably to redesign the safety circuit and implement a modern safety relay that cross checks the contactor operation automatically. Much like Geospark's example above

Safety Circuit Issue

ASF,

I just learned two things:

I did not know what the tail symbols indicated on the contactor coils.
And now it makes sense.

Secondly you obviously are very good at quickly seeing exactly what this circuit required from all contacts at the right timing to work as engineered originally.

I very much appreciate your expertise and advice on this matter.

With brand new original contactor this does work but only for a short time (say 1-3 months), and then fails again. We really have to have this circuit redesigned.

I am grateful again for yours and everyone’s help.

No problem! Yes, I can see how a new contactor might just fluke it for a few months until the linkages got just enough slack in them. But I guess it'll give you a few months to redesign the circuit

Good luck!

Bering C Sparky said:

Not sure if these would be proper to use in a safety circuit or not but most manufacturers sell aux contact blocks with time delays.

You could use the N/C contact on the Aux Time Delay block and the N/O contact on the Contactor itself.
This should function the way you are looking for, N/C would be held for a fraction of a second allowing time for N/O contact to to completely close tight.

Here is an example of one from Schneider.

http://www.alliedelec.com/schneider-electric-ladt2/70007846/

BCS

Click to expand...

Sorry this is definitely NOT a safety device

Search this site for other safety posts - there are many
look at some of the ones I have previously posted in.

if you need time delays - use a safety relay with a Cat 3 safety timer

Yes, that drawing clears things up a lot.

The contactors should never have been used for this safety application. The reason they needed to use conventional contactor interlocking was because they knew that these contactors did not provide force guided, mechanically linked auxiliary contacts.

You really need to get away from these contactors. They are not safety rated and can be too slow opening their main contacts. The 3TH4x contactor relays are in fact designed for railway applications, where you can have fluctuation in supply voltage and ambient temperatures. Depending on the voltage being under or over, the make/break times can vary significantly. If you note the Varistors across the coils. These are for overvoltage protection which tends to be an issue in railway applications because of distances, etc. This feature adds time to the opening of the contacts. I'm not talking about them not being suitable fast enough to prevent your chattering problem, I'm saying that they may not be opening their main contacts quickly enough to de-energize the hazard in the required time. They are not force guided and mechanically linked and so should not be used in a safety system.

Why force guided?

All contacts on a safety rated contactor are force guided, that is why the contacts are built-in and not removable. They are mechanically linked inside to the main contacts, so they all operate together, under force, when the contactor coil de-energizes. On safety rated contactors, the NC contacts will not close, and the NO contacts will not open, if a main contact has welded. So, by using the proper safety rated contactors you do not need to wire it as per the original drawing, but more like the example I provided. When the safety device is actuated, if the NC contact on one contactor is not forced closed, then there is most likely a welded main contact. That is why the second contactor is there, to provide a redundant method of de-energizing the hazard. With the NC contact held open, the safety relay cannot be reset.

Regards,
George