Solid State problems

Hi guys,

Here' my situation,
I'm using a PILZ PNOZ multi as safety PLC to control all safety related components in a production unit. 2 relay-outputs of the PNOZ are used to switch the enables of a FESTO SEC-AC servo drive.
The SEC-AC is placed in another cabinet about 20M further. Because of dithering the 2 relay outputs of the PNOZ will switch 2 solid state relays in the next cabinet, these solid states will enable the drive.
The problems is that when I reset both PNOZ outputs, the solid state will still transfer the load to the SEC-AC. So the 24V will still be comming thru to the enable inputs. I measure about 0.4V on the input of the solid state, the LED will not show that it is switched on. When i take the solid state out of the housing or disconnect the cable comming from the PNOZ the load goes down, so there's no problems with the housing. One option would be to use a normal relay in front of the solid state but I'm sure there's another sollution such as a transistor. Who could give me another option and maybe explane why the solid state works this way?
Thx,
H.

Try http://www.pilz.com
Opto-couplers may prove a better option than SSRs. SSRs have small off-state leakage currents.

Leakage current comes from the snubber network. The amount of leakage current (typically 10-15 mA) is dependent on the size capacitor used in the snubber network. Some SSRs use inductive snubber networks instead of resistor-capacitor networks, which will have a typical leakage current of less than 1mA, see Crouzet.

Shunt resistance?

Small Current Loads:
Small current or high sensitive loads give some troubles such as no SW OFF even by cut off input voltage or also by off state leakage current during open circuit of SSR. In that case please adjust the level not to keep SW ON by shunt resistance RP in parallel to the load in order to reduce the off state leakage current through the load.
Resistance RP is calculated by the following formula. An approximate above value is the same value as load resistance RL.

Dear Heino,
The other side of story is if such a solution can still be called "safe".
As far as I remember codes say something about "positively driven electro-mechanical contacts" when it comes to safety.

What you are doing is a not quite right. The relays that are controlling the device should have positively driven auxillary contacts that are monitored by the pilz. If your SSR fails the Pilz does not know and therefore cannot shut the system down.
Regards Alan Case

Hi,
Thx already for your replies, about the positive driven contacts. This was my first option but didn't work. With both relay or transistor outputs directly from the pilz the drive wouldn't enable. That's why we tried the SSR, but youre right this isn't the way to work, I've contacted Sick about the problem.
Silva Foxx, thx for your explanation.

After re-reading your original post, you need to do a risk assesment of the machine which will tell you what level of isolation is required.
From my understanding you are using the PNOZ multi to control power to the enable of a drive. Very few drives are able to be safely isolated this way. (AB powerflexes with the option card can but only to cat 3) The PNOZ multi is usually used to control two in series safety contactors that break the main power to the drive. The safety contactors have positively driven auxillary contacts that feed back into the PNOZ to indicate both contactors have opened. If 1 has welded in then the system will not enable again until the contactor is repaired.
Controlling the enables of a drive still means power is applied to the drive and an internal fault can start the drive.
Regards Alan Case

Hi Alan,
We've done a risk assesment and when production start I will make sure that all requirements are fit.
Your right about the the 2 contactors that break the main power, we are using this method. The reason why I want to break the enable is because 2 times every cycle, the operator should have acces to the drive when its not in motion. When I taken away one enable the drive is safe to gain access to, light curtains (cat.4 & 2) are used to secure access when the drive is enabled.
During startup I'm experiencing lots of problems, due to this I'm running short in time, that's why I'm using the SSR's. Your right that's not the way to work, but while testing I'm only working 1/10 of max speed and the light curtains are configured to switch of the contactors when I walk in with an enabled drive.
Thx for your interest.
Heino