Safety contactors in front of VFDs

[milmat1]

I have a project where the customer specs will not allow the use of the VFD's STO safety function. They demand dual safety contactors in front of the VFD's.


They sent some sample drawings of their existing machines.
And they have installed dual Schneider contactors in front of each VFD as mentioned. However, they are using a LC1D09BD for example, which is not a safety rated contactor.



Is this a standard practice elsewhere(Australia, because they have dual contactors ? Can we use this line of contactors as such a safety device ??

 

[joseph_e2]

I'm not a certified safety engineer, but...

...what's the point of using contactors that aren't safety rated? I would think that the drive's properly tested and certified STO function is more reliable than a standard contactor.
The VFDs I've worked with are rated up to category 3 and PLd with their built-in STO. The addition of a single properly-rated safety contactor (they almost always say to put it on the LOAD side) brings them to category 4 and PLe.

Has the customer explained their rationale, other than "Just use this"? IF that's what they require, and you get that in writing, it's on them. But I'm not a lawyer either.
Honestly, I would never put a contactor on the line side of a VFD. I've seen it done once as part of a safety circuit for a servo drive that didn't have a proper STO function...and it killed the drives quickly. We had about 12 systems in service and we were repairing 2 or 3 drives a year with burnt input stages. Usually the inrush resistor would pop, often taking other stuff with it. Maybe their experience is different, but that seems to me to be the wrong approach.

 

[milmat1]

I'm not a certified safety engineer, but...

...what's the point of using contactors that aren't safety rated? I would think that the drive's properly tested and certified STO function is more reliable than a standard contactor.
The VFDs I've worked with are rated up to category 3 and PLd with their built-in STO. The addition of a single properly-rated safety contactor (they almost always say to put it on the LOAD side) brings them to category 4 and PLe.

Has the customer explained their rationale, other than "Just use this"? IF that's what they require, and you get that in writing, it's on them. But I'm not a lawyer either.
Honestly, I would never put a contactor on the line side of a VFD. I've seen it done once as part of a safety circuit for a servo drive that didn't have a proper STO function...and it killed the drives quickly. We had about 12 systems in service and we were repairing 2 or 3 drives a year with burnt input stages. Usually the inrush resistor would pop, often taking other stuff with it. Maybe their experience is different, but that seems to me to be the wrong approach.

Exactly my point. They are quite strict, and we get the "because we said so" type of responses.


My view is the VFD manufacturers went through a lot of testing and trouble to get the STO function certified for PLe / SIL3 safety, Why wouldnt we want to use it..

 

[joseph_e2]

Yeah, I just don't get it...

 

[parky]

Very often, this is used & the N/C contacts are seriesed up & fed back to ther safety relay, i.e. if any one contactor welds in then the safety relay cannot be re-energised, two contactors are unlikely to weld in at the same time. This has been used for many years. In many systems I have seen a group of VFD's or motors have a common contactor feeding individual contactors for each motor/VFD, It is essential that the N/C contacts be put in series as feedback to the SR to enable the safety relay to be operated.
In this day & age with VFD's that have the tech I am surprised that they have not moved on. I have been out of the loop for a few years now so do not know the current regs on this.

 

[joseph_e2]

Right, but he said that the customer has specified contactors that aren't safety rated, so there's no guarantee that the NC aux contacts are following the main contacts.

 

[Rob...]

All contactors have a safety rating....

It maybe just that they aren't rated high enough for your application.

 

[dummy_bit]

I'd just send back how the contactors not being safety rated negates their safety categorization of the machine. I've seen older VFDs use dual contactors, but newer VFDs (PowerFlex 525s) we always use their incorporated Safety Torque-Off function. Why add in more parts that are not necessary?

 

[parky]

In my day, we grouped the first contactor in say 3 starters sized to above the total current rating of that group these were started in sequence so the total inrush was well below the rating of that group, the second contactor for each motor/vfd was also sized to be above the current rating of that drive.

 

[ndzied1]

On the data sheet the contactor has a Safety reliability level
B10d = 1369863 cycles contactor with nominal load EN/ISO 13849-1

So I'd say technically it is safety rated.

 

[L33er]

On the data sheet the contactor has a Safety reliability level
B10d = 1369863 cycles contactor with nominal load EN/ISO 13849-1

So I'd say technically it is safety rated.

Good point! With B10d = 20M cycles contactor w mechanical load, that translated to MTTFd = 2987 years.

The only issue left is w the drive power up/down. #1. slow recovery time.
#2 need to have an AUX contact to shutdown the drive properly to avoid drive damage.

 

[lfe]

I used Schneider LC1D09 contactors as safety contactors, since they have the contacts mechanically linked so an NC contact will never close while another NO remains closed.

 

[Puddle]

In effect, all STO does is to cut power from the drive and coast to a stop. Actively braking the motor to a standstill is not done by STO, so functionally there's no difference between using contactors or STO.

I'm assuming their decision has come from either blindly following the wiring diagram that came with their safety controller, or from older designs where VFDs are not used.

 

[L33er]

The is a big diff between using STO and Contactors.
Certain company requires that no high voltage at the device and that what the contactors do.

 

[kolyur]

I used Schneider LC1D09 contactors as safety contactors, since they have the contacts mechanically linked so an NC contact will never close while another NO remains closed.

Agreed, we have been using the Schneider LC1 contactors for years in safety applications.