safety relay module, K3 and K4 coils

rbrower

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Feb 2021
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canton, SD
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looking at a safety relay module, K3 and K4 coils; do these relays need to be force (positively) guided, or can they be just regular relays? it does not not drive a motor directly, just the solenoid valve to the hydraulic valve on the motor.

safety relay wiring FSD OSSD EDM K3 K4 contacts 4-21.jpg
 
What type of components you use would have to be determined by a risk assessment.

What the force guided relays gets you is the NC contact (that can’t be closed if the NO contacts are closed) the as feedback for proper operation.

SRPCS as a risk reduction measure is really about meeting the required reliability of the system. But the required reliability of the system is determined by the risk assessment.

What does your risk assessment say you need?
 
i'll have to get with my safety director to hash that out. the system has a moving ram behind a light curtain. the system is already in place, i am just doing some maintenance to it and was curious. the current relays aren't force guided.
 
rbrower said:
looking at a safety relay module, K3 and K4 coils; do these relays need to be force (positively) guided, or can they be just regular relays? it does not not drive a motor directly, just the solenoid valve to the hydraulic valve on the motor.
Click to expand...

Yes, they need to be force guided, auxiliary NC contact must close only if the main contacts are opened. In accordance with IEC 60947

Schneider basic contactors series LC1D... are OK for that.
 
rbrower said:
i'll have to get with my safety director to hash that out. the system has a moving ram behind a light curtain. the system is already in place, i am just doing some maintenance to it and was curious. the current relays aren't force guided.
Click to expand...

This question can’t be answered in any generalizations. The other comments saying they have to be force guided or that you have to monitor the valve are both incorrect.

You may very well have to do one or both of those things. But that would be determined by a risk assessment.

What the standards say is that a risk assessment must be performed and if the level of risk is not acceptable, the risk must be reduced to an acceptable level. Some of those risk reduction measures may include SRPCS and, if so, those measures must meet reliability requirements (which vary depending on the level of risk as determined by the risk assessment).
 
older systems may not have the guided safety relays, BUT newer systems Must have a safety relay. i don't have my books available with me.
the safety relay which has guided contacts helps prevent chattering, contact arcing, and welded contacts. it will then energize a MCR relay to power the system.
As already stated, you need a risk assessment for any upgrade. this will involve engineering, safety, and maintenance so you can discuss the existing equipment and operation, the proposed changes, and possible risks.
if i am incorrect, someone please correct me.
james
 
I think there is some confusion here the safety relay (relays) should be force guided, however, the OP's question is on the contactors K3 & K4, these are not part of the safety relay, there are two main plus motor drive contactors, their purpose is if one welds it's contacts then it is unlikely the other will hence the feedback to the safety relay so if one of the main contactors welds in then the safety relay cannot be re-energised, been a couple of years since I have done any safety related designs so things may have changed. it was common practice to use one main contactor feeding a group of motors (or other moving equipment) and then a dis-connect contactor for each motor, all these are fed back in series as N/C contacts and in the event of a single or multiple contactors welding in then the safety relay cannot be energised.
 
parky said:
I think there is some confusion here the safety relay (relays) should be force guided, however, the OP's question is on the contactors K3 & K4, these are not part of the safety relay, there are two main plus motor drive contactors, their purpose is if one welds it's contacts then it is unlikely the other will hence the feedback to the safety relay so if one of the main contactors welds in then the safety relay cannot be re-energised, been a couple of years since I have done any safety related designs so things may have changed. it was common practice to use one main contactor feeding a group of motors (or other moving equipment) and then a dis-connect contactor for each motor, all these are fed back in series as N/C contacts and in the event of a single or multiple contactors welding in then the safety relay cannot be energised.
Click to expand...

In my opinion you are correct. No need, in this instance to be force guided. K3 and K4 are nothing more than 2 motor contactors in series.
 
parky said:
I think there is some confusion here the safety relay (relays) should be force guided, however, the OP's question is on the contactors K3 & K4, these are not part of the safety relay, there are two main plus motor drive contactors, their purpose is if one welds it's contacts then it is unlikely the other will hence the feedback to the safety relay so if one of the main contactors welds in then the safety relay cannot be re-energised, been a couple of years since I have done any safety related designs so things may have changed. it was common practice to use one main contactor feeding a group of motors (or other moving equipment) and then a dis-connect contactor for each motor, all these are fed back in series as N/C contacts and in the event of a single or multiple contactors welding in then the safety relay cannot be energised.
Click to expand...

If K3 and K4 are not force guided, then their NC contacts do not guarantee the NO contacts are all open.

Redundant contactors don’t do you any good if you are not correctly detecting component failures. Monitoring is introduced in circuit structures before redundancy is (CAT 2 vs CAT 3).

What they need to be depends on the risk assessment.
 
I’ll also add that if the requirement is PLe, they may not only have to be force guided redundant monitored contactors, but they may have to be from different manufacturers to reduce CCF.
 
This:
jhenson29 said:
If K3 and K4 are not force guided, then their NC contacts do not guarantee the NO contacts are all open.
Click to expand...
+1
Most contactors are force guided. So it is not a big problem.
But you have to check, since it is most but not all. There was a thread a year ago or so about an AB contactor why there was an advisory note when purchasing, that its contacts are not force guided. This is the reason for that advisory note.
 
In a hydraulic system, stopping the pump or a solenoid may not be enough since it does not imply that the cylinder will always stop, it is necessary to evaluate and prevent other risks, such a hose breaking
 
lfe said:
In a hydraulic system, stopping the pump or a solenoid may not be enough since it does not imply that the cylinder will always stop, it is necessary to evaluate and prevent other risks, such a hose breaking
Click to expand...

You are correct that stopping the pump or solenoid may not be enough.

But a hose breaking is not itself a risk. It’s a source for hazards.

What (if anything) you have to do about it depends on the risk assessment.

It’s not necessary to prevent risks. It’s necessary to reduce the risks to an acceptable level.
 

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