Interlock device - only two channel?

[rQx]

Hi,
Had a discussion with a cunsultant about safety and he claimed that for example an Allen Bradley MC1 (two contacts) magnetical non-contact interlock switch is only allowed to be connected using both channels and NOT in a single channel system like PLc. Its B10d rating should only be for dual channel system and if you install it single channel you are not connecting it according to the manual. I think he is wrong, and also in the VDMA library for sistema is says.

2) This product has two electrical switching functional channels. The B10d data given for this product is based on a failure of either channel, which can be used to determine the MTTFd of a single channel.

Any input on this, or where he might have gotten this from? He says this applies to ALL interlock switches that have two channels

 

[ASF]

I think he is wrong too, and you've already found explicit proof.

 

[I_Automation]

I had a situation replacing a magnetic switch used for turntable in position A or B.


The customer had a brand new A-B 2 channel interlock switch and mated magnet that we both figured just use one channel so the PLC knows turntable A is IN.


The switch wouldn't activate only channel A.
Then if the 2 channels were wired in series to use both, the switch wouldn't work because channel B was not receiving a voltage when channel A wanted to activate.


Also, giving both channels 24V power didn't work because then channel B detected a wire break.


Had to wire both channels to power and a second PLC input and run another cable to get this magnetic switch to work.

 

[parky]

According to Pilz a dual channel system can be wired as single channel, there are plenty of diagrams showing two & single wire modes, so I think he is wrong, if the system was designed as a single channel then there is no reason that a dual channel sensor or safety relay that is dual channel cannot be configured as a single channel, we used many of the AB stainless dual channel (2 N/O + 1 NC) in single channel mode what we actually did was wire both in series just as extra safety. Now if Pilz say you can use a dual channel as a single channel then providing you are not trying to change an existing system from dual to single.

 

[GaryS]

he is correct to get the safety rating you need to have 2 channels into a safety controller or relay, that's to insure that it working correctly
if you fail to connect things correctly then if somebody get hurt it on you
if you don't understand and know what you are doing with safety systems then you should not do them and ask for help

 

[parky]

I dissagree, see below this is from AB
Providing the replacement is rated to the same spec or above and the sytem has been risk assessed for single channel then it is ok, however, under the most recent guidelines dual channel is certainly more common but nearly all safety dual channel relays & switches can be used as single channel.

 

[A_G]

I think there are two questions 1) will it function electrically using only one channel, the answer may be yes 2) will it meet required safety standards using only one channel. If you are using outside consultant ask them to find the exact section of the electrical code or safety standards document that requires this, and that way you know for sure

 

[rQx]

Thanks for all the input