Wiring Motor Safety Contactors in Series

I have three Banner safety relays controlling three AB safety motor contactors. I want to wire the 'T' terminals on the first contactor to the 'T' terminals on the second contactor and then wire the 'L' terminals on the second contactor to the 'L' terminals on the third contactor and the 'T' terminals on the third contactor to the motor. THe drawing will be documented as such. Would this be a problem?
 

paulengr

Senior Member
Looking at Banners site I don’t see T or L contacts,only Y and not numbered. AB sells safety relays (700 series) but a safety contactor doesn’t sound right.

Perhaps explain conceptually what you are trying to do and with what parts? AB has several different relays from 70” series force guided relays to Minotaur safety systems to safe torque off in drives. Banner is equally loaded with many different products. I have never heard of.a “safety contactor” and almost by definition since contactors are subject to contact welding a safety contactor sounds like an impossible device.
 
Thank you for the response. I am referring to the AB/Guardian Safety Contactors where the 3 phase wires connects to the the L1, L2, L3 terminals and T1, T2, T3 connects to the load i.e. motor in this case. The part number is AB 100S-C09EJ23C 9A I have attached a picture of the terminals.
 

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sii

Senior Member
Location
Nebraska
It sounds to me like you are creating a series of motor contactors that requires all three to be pulled in for the motor to run. I assume you are intending to control each contractor with its own control device, whatever that might be. Typically you would see the control devices wired in series, then connected to the coil of one contactor. Multiple contactors creates multiple points of failure that are just unnecessary.

While it would work, it certainly sounds like an awkward way to go about it. If I’m misunderstanding your intent please correct me.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
It sounds to me like you are creating a series of motor contactors that requires all three to be pulled in for the motor to run. I assume you are intending to control each contractor with its own control device, whatever that might be. Typically you would see the control devices wired in series, then connected to the coil of one contactor. Multiple contactors creates multiple points of failure that are just unnecessary.
Agreed.
 

Russs57

Senior Member
I don't see redundancy the way you have explained it. I see quite the opposite.

If you want reundancy you first need to decide if is a manual operation or an automatic one. Automatic runs the risk of the second contactor/starter being exposed to the condition that ruined the first one. Manual would mean an interupption of service. Pick your poison. I would go manual.
 

WasGSOHM

Senior Member
Location
Montgomery County MD
Occupation
EE
Assuming contact closings are independent events:

If you want reliability in opening contacts, only a single "juror member" of the three would have to vote to "acquit".

If you want reliability in closing contacts, all three juror members would have to vote "guilty".

You can't minimize both erroneous judgement calls at the same time.



It is unbelievable how many supervisors at NASA don't get this, and NASA is supposed to be about reliability.
 

paulengr

Senior Member
Agreed although there is a mix. Speaking of inputs for instance if we go from 1 to 2 input devices when there is disagreement with the inputs we cannot determine which if any has failed. We can either logically AND or OR the results. One increases reliability, the other safety but they are mutually exclusive. With 3 inputs we can do best 2 out of 3 which increases both safety and reliability.

Speaking then to contactors you will find looking for instance at ORIEDA or perhaps better data that failure rates are in the range of 10^-6 or even lower. So you are already near or at SIL 3 range and the best you could hope for is SIL with say Triconex style implementation which is way beyond most equipment capabilities (triple redundancy or better) and usually points to a safety design mistake. Before you go there look at the failure rate of the load. With pumps, compressors, etc. barely making 10^-5, you need two or more process equipment components so redundant contactors is pointless. To meet SIL 3 or 4 though there is minimum redundancy.

I’ll leave you with this. Frequently hospitals and water plants order VFDs with bypass contactors. These designs are a historical holdover from the 1980s and earlier when drive reliability (to say nothing of DC motors) was atrocious. So they add three contactors. One on the line and load side of the VFD to isolate it and a third as the bypass. There are two problems here. First we now have 3 components in series on the VFD side any one of which can cause the VFD side to fail. Reliability of modern VFDs isn’t that much lower than contactors so the effect is to lower the reliability of the VFD path significantly. Second the controls are more complicated further lowering reliability to well below that of a single contactor even if there are redundant paths. These same operations typically have 2 or more redundant process equipment systems in parallel so adding bypass contactors is counter-productive. The final issue is that the vast majority of these systems do not have a control damper/valve or it is completely inadequate or not implemented to the point that they can’t run in bypass in the first place so it’s not truly a redundant solution.

Looking at your issue obviously you would just have two complete starters (disconnect, short circuit protection, overload protection, contactor) in parallel for redundancy but if you just parallel them you will nit for instance be able to shut it down on welded contacts so you have to series contactors so again...triple parallel starters with series contactors at a minimum.
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
It sounds like he is more worried about the contactor getting stuck closed and allowing the motor to run when it should not be running. I am assuming this is why he is wanting to wire the load in series through the contactors. If one contactor fails and gets locked in the closed position the other two would still open and not allow the motor/device to run. It would work assuming the contactor coil is getting the proper voltage and would only protect against the contacts welding shut/closed. It may make reliability a little worse as now all three contactors would have to work properly each time for the load to run. If using a control transformer for the controls/coils of the contactors, keep in mind that you just tripled the size of the control transformer needed not only for the holding VA of the contactors but the inrush VA of the contactors as well.
 

paulengr

Senior Member
It sounds like he is more worried about the contactor getting stuck closed and allowing the motor to run when it should not be running. I am assuming this is why he is wanting to wire the load in series through the contactors. If one contactor fails and gets locked in the closed position the other two would still open and not allow the motor/device to run. It would work assuming the contactor coil is getting the proper voltage and would only protect against the contacts welding shut/closed. It may make reliability a little worse as now all three contactors would have to work properly each time for the load to run. If using a control transformer for the controls/coils of the contactors, keep in mind that you just tripled the size of the control transformer needed not only for the holding VA of the contactors but the inrush VA of the contactors as well.
There are calculations for contact welding where contacts are purposely oversized, basically an offshoot of the safety relay standards. You can reduce but not drive the probability to zero. Second NEMA ICS 2 and the IEC equivalent have installation rules for type 2 (no damage) starters which essentially means using a low peak let through fuse such as an RK-1 sized fairly tight. It won’t stop welding but it helps. The IEC documents cover going to say AC-4 service (upsize the contactor). Then looking at the MBTF charts for that classification specifies the failure rate. The inherent issue of course is unless TuV third party certifies it the failure rate is probably not very credible.

My point is usually what I find is that you put all the analysis into the sensors, PLC, and so forth but invariably the weak part of the system is ALWAYS the outputs. Valves are usually 10^-2 failure on demand. Pumps, compressors and so forth maybe 10^-3 to 10^-4. So it usually doesn’t matter how reliable the upstream is.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
For those unfamiliar with this concept in Machibe Safety systems, some safety regulations require redundancy of power contacts to achieve the highest level of safety, and the contactors must be specifically approved “Safety Contactors” with what are called “force guided contacts”. Those contactors are NOT used for the normal On-Off of the load so they are not subjected to normal wear and tear of opening and closing under load, the control and safety system will operate them unloaded UNLESS there is a safety emergency. For the some of the highest Safety Integration Level (SIL) category, you must have two Safety Contactors in series, so adding the contactor that actually controls the load, you end up with three.

They would be wired T to L to T to L, not T to T and L to L.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Looking at Banners site I don’t see T or L contacts,only Y and not numbered. AB sells safety relays (700 series) but a safety contactor doesn’t sound right.

Perhaps explain conceptually what you are trying to do and with what parts? AB has several different relays from 70” series force guided relays to Minotaur safety systems to safe torque off in drives. Banner is equally loaded with many different products. I have never heard of.a “safety contactor” and almost by definition since contactors are subject to contact welding a safety contactor sounds like an impossible device.
Basically safety contractors are regular positively guided contactors that are painted red. there's some other minor differences but that's the big deal as far as I can tell. I think they remove the button that allows you to manually override it as well. IEC relays are all positively guided so that all the contacts are tied together so that if you get welding they all stay together so it can be tested by looking at an auxiliary contact. In extreme cases where you need a high level of likelihood that a circuit will work when the eStop is tripped, you can put two contactors in series. Usually this is done in the feeder circuit of multiple motor starters if this is needed.
 
For those unfamiliar with this concept in Machibe Safety systems, some safety regulations require redundancy of power contacts to achieve the highest level of safety, and the contactors must be specifically approved “Safety Contactors” with what are called “force guided contacts”. Those contactors are NOT used for the normal On-Off of the load so they are not subjected to normal wear and tear of opening and closing under load, the control and safety system will operate them unloaded UNLESS there is a safety emergency. For the some of the highest Safety Integration Level (SIL) category, you must have two Safety Contactors in series, so adding the contactor that actually controls the load, you end up with three.

They would be wired T to L to T to L, not T to T and L to L.
For my own reasons why not T to T and L to L? BTW... my mistake. I should have never said three safety contactors. It's actually only two safety contactors.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
Assuming contact closings are independent events:

If you want reliability in opening contacts, only a single "juror member" of the three would have to vote to "acquit".

If you want reliability in closing contacts, all three juror members would have to vote "guilty".

You can't minimize both erroneous judgement calls at the same time.

It is unbelievable how many supervisors at NASA don't get this, and NASA is supposed to be about reliability.
Watch your glottology dude. . . the fairy godmother is watching from her rocking chair :) :)

I concur with your "JUROR" analogy though.

My former office-mate still works at NASA.

Voting logic as applied to redundancy-- is a legitimate engineering sub-category to ensure and/or monitor operation without violating specs limitations.

Although, mostly a "device" for configuring computer systems, voting logic can also be used in non-computer systems similar to OP's post.
His design involves contactors which I would call MAG STARTERS instead of relays. I don’t endorse however--to deploy such design (although possible) . . . .it seem like “horse and buggy” era of systems control design. (if ever there was such a thing during Buffalo Bill’s days).

Magnetic starters have "T" and "L" terminals. . .not on relays. :)

A more reliable form of voting logic involves several devices . . .usually a minimum of three.
The performance of say: a three-device setup are compared by voting logic.

A disagreement of the three will disable an output but continues to work if failure occurs in one of them.
This is a standard technique in avionic systems design. . .and safety margin in the event of critical system failure.

This is an important feature in the operation of the Space Shuttle.

A –HAH! . . . There’s NASA.

System failure in navigation while orbiting the Earth could throw the space shuttle off its asynchronous orbit-- into the vast region of deep space.
And no one will ever know where they will end up. :(

If the shuttle does survive in the tugging and pushing of other celestial object and eventually die. . . it will a be fun toy for the Moon and the Sun until captured by the Sun's gravity.
Some of its parts assuming everything will not melt --will orbit round and round the Sun.
 
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