TroubleShooting Hardwired Daisy Chain Controls

We have some old lines that have many door and cover switches integrated as part of the estop circuit. Some of these circuits have 60+ devices Limit switches,door switches,estop buttons,etc.

I have one that has been giving me a fit the last few days as it will intermittently stop the line but the fault does not stay and i really hate to start jumping out safey switches in a large fashion but this is costing a lot of down time which is a lot of money some of thes come back to a common terminal strip but many go straight from one device to another.

Is there any test equipment with multiple channels that can detect which device is lost first when they are all on a common feed?

Any other ideas on the best way to find this?

If the devices are in series then you can use a voltage meter to break it down.

Hook your meter across the center point of the circuit and the common. You should now see whatever voltage is used in your safety circuit. Set your Min Max setting on your meter. It should read you saftey voltage for the max and the min at this point.

Now wait for the next trip. If your Min is still the voltage you saw when you first hooked it up then the problem is "upstream" from this point. If your voltage dropped then the problem is downstream.

You can keep doing this until to narrow in directly onto the troubling device.

An old thread that may suit.

Take a look http://www.plctalk.net/qanda/showthread.php?t=35449&highlight=tattle+tale


PS: How do I replace the link with here?

I remember seeing this in the Panalarm Series 90 annuniciator catalog years ago, where several points monitored contacts wired in series. A few years ago, we built our own using a MicroLogix 1100 to monitor a half dozen or so contacts. Connect the load side of each contact to a discrete input. Write a program with a rung for each input that, when the input turns off and the common First Out bit is off, latches the contact's alarm bit and the First Out bit. The rungs must be in the same order as the contacts, since all inputs after the open contact will be off. The First Out bit keeps them from latching their alarm bits.

Edit: While I was writing this, silva.foxx found an old thread with the same idea.

[URLx="www.google.com"]Silva.Foxx[/URLx]

Remove the x's from the url bit's.

here (testing: same link as above so please ignore)

Thanks HJTRBO

Well i found the ofender but it too is a mystery. This system was installed when the plant was built so it is over 40 years old. A plc was added years later but the switches and wiring is still used.

In this cabinet the safety chain passes through the run relay for this aux equipment and that relay is what is dropping out.

What doesn't make sense is this relay is just a standard start/stop circuit and it is a 3 pole relay using the center pole N.O contacts as the seal in. Every few minutes this relay just drops out with no change in load or anything so installed a new relay and base,checked all the wires and replaced a few and it is still doing it? I even put a jumper clip across the stop button in case the contacts on the stop button were the problem and it still happens.

So next i said i must be losing the neutral (120 vac circuit) nope i installed a new neutral wire directly from L2 of the realy base to the neutral/ground bond on the 480 x 120 control transformer. Still happening

The only thing left that i can think of is some kind of inductive kickback? But the thing is nothing on this system switches on or off once this system is running. It does have a 150 HP motor that is ran from a reduced voltage starter / Auto transformer starter. The coils for both those contactors are on the same transformer and neutral system.

But the relay i am having issue with does not power on until the 150 motor is up and running but i bypassed that also. This relay is running now from the most simple text book start/stop with seal circut there is and the coil still drops out.

Voltage is perfect never dips.

Describe the whole circuit that runs that relay.

Sounds like you are using a cube relay. I would try AB 700-CF or similar type.
We used to have light curtains that were interfaced with cube relays (not the best solution in terms of safety). After I installed force guided relays problems went away.
The salesman of the light curtains called it a relay chattering.
Tom

Well, did you change the relay? Contacts do fail over time.

The Plc Kid said:

Voltage is perfect never dips.

Click to expand...

How and where are you measuring this voltage? At said relay coil?

I think the time has come for a wiring upgrade to bring this kit into the 21st century.

rdrast said:

Well, did you change the relay? Contacts do fail over time.

Click to expand...

Bob

I changed the relay and the base. It is setup pretty much like the first example on this link http://drstienecker.com/tech-332/5-ladder-logic/

Text book basic. There are other permissives in the circuit but they are all jumped out and even the contacts on the stop button are jumped. So we have L1 going to one side of the stop button coming out of the stop button we next go to the start and out of the start we go to the relay coil.

The circut works correctly it is just that after a few minuites and sometimes a hour it will drop out. I know the control voltage is not the issue because it we lost the voltage other parts of the circuit would drop out but they remain running.

As you have replaced the relay and bridged the stop button the fault must be in the wiring to and from the buttons. Check all your terminal connections, if the stop/start buttons are remotely positioned you could put these buttons temorarily on the control cabinet to prove the fault and then replace/fault find the wiring later, without losing the integrity of your safety circuit.

I was thinking of adding a mov but i only have 3 mov in the house. They have no voltage markings on them. Is there any way to tell the value if it is not marked? I don't think so but i thought i would ask.

The mov has a part number of 130L20 and not much turns up on the internet. Is the 130 part of the number the voltage rating perhaps? if so 130 vac this will work.