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Safety Relays & Contacts in Series

Muffler Bearing

Aluminum
Joined
Jan 19, 2010
Location
KY, USA
I'm always trying to improve my knowledge on one topic or another. Lately, I've been reading a bunch about safety relays. My question is this - if you have one circuit that has the same purpose ("E-stop!") .. can the contacts be in series with one another? Note - this is not a question specifically for any particular machine.

Most manufacturers literature has the same variation of wiring diagrams. For every safety relay channel, there's always one e-stop NC contact, wired into the coil terminals of the safety relay, and then a monitoring loop contact that either has an identical contact or jumpered for automatic restart.

If you have 10 e-stop buttons, 2 pull cords, 3 door switches, each with two sets of NC contacts, what would be the problem with having one set of contacts of each device in series for the relay coil, and the other set on the monitoring circuit?
 
If I understand the question....... That's the way it's usually done. E-Stop is usually one loop encompassing all of the E-stop buttons, etc.
 
The E-stop function normally uses one and only one normally-closed loop, which can have any number of switches, relay contacts, or other interrupt devices.

The reason is simple...
When implemented this way, any circuit failure such as a broken wire will force a STOP condition.
With any other configuration a broken wire could defeat the E-stop function completely.

- Leigh
 
Leigh has the right approach - single loop connected to a relay that will not energize when the E-Stop loop is restored. This is the tried and true approach that up until the last 5 years or so was accepted practice.

Today - (and in fact the rules changed in December, 2011) - you would use both contacts of each of the safety devices in a parallel loop. In addition, contacts would need to be forced open by actuation of the safety switch (if a mechanical PB or pull-cord) such that if they ever welded shut, that the contacts would be broken open through mechanical means.

Modern systems have monitored safety loops that are managed by two separate monitoring devices (Safety Relays or Safety PLCs) that compare solutions to ensure safety integrity. Both loops are pulsed with current to verify continuity (and agreement) every so often (on the order of 2-3 times a second).

Latest Safety / E-Stop design practices complying with OSHA on new machinery are defined by ISO-13849-1 and IEC 62061 . . . lots of fun navigating through what this means for machinery today.
 
All good stuff except.....

Sometimes and perhaps a lot of times, the final item in the safety loop is the power switch or contactor. If it is a mechanical contactor held closed with a magnet system, then it is vital that it opens when called upon to do so. Things that I have seen happen in past years. Magnets with no air gap, contamination of the pole faces with sticky stuff like grease or oil, mechanical binding or contact welding. Story's like the bridge crane that run off the end of the track because the contactor stuck shut. On the biggest issues I encountered was and is maintenance people lubricating a magnet to keep it from rusting. This is also a HUGE issue with contactor manufacturers as to how to protect the pole faces from rusting without increasing the noise or causing sticking issues. When I designed the GE 300 line, I found that in order to get the mechanical life from the switches, that lubrication of the magnet system was needed. I went to great lengths to verify that the system I had chosen could not cause magnet sticking. The same type of thing with contactors made by Furnas Electric, that a corrosion inhibitor used on the pole faces could not cause sticking.
 
All good stuff except.....

Sometimes and perhaps a lot of times, the final item in the safety loop is the power switch or contactor. If it is a mechanical contactor held closed with a magnet system, then it is vital that it opens when called upon to do so.

That is a great observation - and in recognition of this, safety contactors are dual circuit devices in safety rated systems now . . . i.e., the correct safety rated solution is two contacts in series.

Here is a single safety rated contact that goes in front of a large winder drive for one of our customers.

CIMG5836.JPG
 
Does that mean there is some kind of monitoring for mecanical failure of the contacter ie a welded condition.

It wouldent be evident until the failure of the second contacter in the series.

I would like this on the up function of my overhead crane.
 
Safety rated contactors have built in monitoring and built in dual contacts in series. The above photo is of a pair of non-safety rated contactors that have been configured to comply as a safety rated power contactor.

From the description of a safety rated contactor from Rockwell / Allen Bradley

Bulletin 100S-C/104S-C safety contactors provide mechanically linked positively guided contacts, required in feedback circuits of modern safety applications. The mechanically linked N.C. auxiliary contacts will not change state when a power pole welds. In addition, the gold-plated bifurcated auxiliary contacts are ideally suited for low-energy applications or feedback control circuits with multiple series-connected N.C. auxiliary contacts.
 
That's a very expensive safety system. What are those, size 7?

The description of the A-B safety contactor is just the description of a standard contactor. All the contactors I am familiar with have guided contacts with aux contacts that operated off the main switch. The gold plated bifurcated contact is standard for low energy circuits to get contact fidelity.

A better solution would be to use a circuit breaker with a shunt trip. I recognize that shunt trips in general require an action of activate, but they are designed to operate in emergency conditions. I am not sure if there is one that requires coil power to hold the breaker closed.

Another option for heavy current is a vacuum contactor.

The best solution for low power is multiple contactors in series that are checked frequently to make sure they work.

Tom
 
Tom - these safety contactors allowed us to implement a Category 3 safety system with a non-safety-rated 300HP VFD. Each drive on the winder needed one of these setups in front of it in order to comply with OSHA safety requirements.

Here is a simplified diagram of one way to implement . . .
39.jpg
 
So no monitering of the motor circut for welded contacts.

I guess that it would be safe up to the first malfunction which wouldnt be detected and the second would be from the mess left behind.
 
So no monitering of the motor circut for welded contacts.

I guess that it would be safe up to the first malfunction which wouldnt be detected and the second would be from the mess left behind.

No, that's why there are two contactors in series. If one welds but the second opens, the aux contacts on the welded one will not change back.

In any case, something like this should receive routine maintenance and exercise just like a backup power system, and, whenever there is an emergency shut down, the entire safety system needs to be checked.

I am sure motion guru would agree.

Tom
 
Latest Safety / E-Stop design practices complying with OSHA on new machinery are defined by ISO-13849-1 and IEC 62061 . . . lots of fun navigating through what this means for machinery today.

I think there are far, far too many standards out there. What about ANSI B11 and NFPA 79? How in the world would you be able to document a design to all the standards out there?

Do most of your big customers specify "I want compliance to A, B" or do you specify in your quote "Machine will be built to latest XXXX revision standards"
 
The machines are inspected by the local electrical inspector for NFPA 79 / NEC 20xx compliance, we are a UL508a shop and label our own cabinets with quarterly audits from ETL.

The safety directive compliance is specified by the customer and in our case, this is almost always in excess of what OSHA requires. These directives are common for larger companies like Boeing or Kimberly Clark or Proctor and Gamble. ANSI B11 deals with identifying hazards / risk analysis. ISO-13849 has a risk analysis component, but also goes into reliability of safety components and and fail safe safety system design with constant monitoring and interlocking operation of the machine such that if any component of the safety system is detected to be compromised, the system safely shuts the machine down.
 
We use series Estop relays on all our machines. Several problem areas we have had:
E stop buttons that can be bumped by operator during normal operation. Installed A/B guard to prevent.
Mechanical Door/access cover switches that misalign.
PLC programming issues. Currently if Estop is hit or fails, machine shuts down with no clue as to why. I would prefer all power removed to control relays with a FAULT display of "E-STOP X." Can't get site safety person to agree or disagree that this meets OHSA/Corporate standards.
In eight years at current plant, would have hit the E stop once, but machine had already caught fire.
Some days I think OHSA and corporate would be happiest if I was wrapped in 6" of bubble wrap, wearing fire proof, flash proof, non-conductive clothing, a full face shield, triple layer ear plugs, and NASCAR style helmet.
 
Two contactors in series

No, that's why there are two contactors in series. If one welds but the second opens, the aux contacts on the welded one will not change back.

In any case, something like this should receive routine maintenance and exercise just like a backup power system, and, whenever there is an emergency shut down, the entire safety system needs to be checked.

I am sure motion guru would agree.

Tom

Hello, on thursday we had one of our contactors weld shut twice in one day. The line voltage goes through two contactors in series to a 2 HP VFD. My theory is harmonic distortion from the VFD. Have you ever experienced anything like this before?
 
Hello, on thursday we had one of our contactors weld shut twice in one day. The line voltage goes through two contactors in series to a 2 HP VFD. My theory is harmonic distortion from the VFD. Have you ever experienced anything like this before?

I would be wiring the "scram" button/control to a large contactor that shorts the
incoming line to ground, just downstream of fuses.
The fuses won't weld.
 
FYI" The cheap hang yaun VFD's use bad logic for the stop controlsw.. It is factory set that stop is NO the contacts and wires have to be closed to stop the machine. If the stgp wire is broken or the contacts get dirty there is no way to stop it.
Bill D
 








 
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