2oo3 Voted relay for hardwired shutdown


Thread Starter


Presently developing URS for 1002D Safety PLC.
Wish to specify type of relays to be used to implement a 2oo3 voted hardwired zone shutdown that will disconnect active power from the relevant digital output termination cards.

A set of 2oo3 voted relay contacts will also be wired into the PLC logic.

I plan to use 3 HIGH RELIABILITY relays with the contacts wired in a 2oo3 configuration. A test pushbutton will be available on each relay to enable all the NC single pole contacts on any one relay to be tested for welding.

Is there a better way of achieving this??

Can anyone suggest suitable relays??
Allen Bradley bulletin 700S relays are safety rated with guided contacts. AB also has an integrated unit so you don't have to wire it up yourself, but its ampere rating is only 4A, so you'll likely end up using two 700S relays with it
anyways. For this reason I prefer to wire up my own instead of an integrated unit. Some other companies offering these are Banner, Schmersal, Pilz, Norstadt, STI. You can't just use any "highly reliable" relay, the relay also must have guided contacts. This means that on a 4-pole NO & NC relay, if any one NO contact welds, then none of the 4 NC contacts will be allowed to make when the coil is de-energized. If wired correctly, the system cannot be reset unless all 4 NC contacts make. Also, set it up so that one of three relays must cycle on, then off, for the other two to energize. I can send you a couple of sample circuits if you like.

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A Solution:
It is not clear what system/process conditions will trip the 2oo3 relay subsystem.

If these relays are actually trip amps that respond to process sensing or measurement conditions, then instruments that offer Failure Modes, Effects Diagnostic Analysis (FMEDA) data offer the highest proven reliability. Models
are available that sense temperature from RTD or T/C inputs, AC current or voltage, or high level 4-20 mA signals. Trip points are configured and the relay outputs wired in a voting configuration. The test function could be
implemented by using a pushbutton to either short or open the input signal to simulate a fault. The relay outputs can be DPDT to both switch the PLC outputs and provide dry contact inputs to the system to provide status.

The Moore Industries model SPA Site Programmable Alarm family offers the PFD data to support SIL calculations to verify suitability for the application.

For more information go to www.miinet.com

Bud Adler
Director, Business Development
Moore Industries-International, Inc.
The Interface Solution Experts
[email protected]
Office: 407.324.9389
Cell: 407.353.2727
Fax: 561.828.2232
Hi Bud,

Thank you very much for your time. Apologies for my lack of clarity. The PLC will be implemented as a 1oo2D HIMA Emergency Shutdown System. The final elements will be motor ESD circuits and ESD valves.

Essentially, multiple Key-release pushbuttons and the PLC watchdog timer will be wired in series to normally energise the 2oo3 voted Master Trip Relays. The 2003 voted contacts on the Master Trip Relays will be used to disconnect all active 24vdc from certain nominated digital output cards.

So essentially, I should be looking for 3-relays having certified FMEDA data.
Much appreciate your help and time. Let me briefly develope my intention and see what you think!...

The objective I have described to Bud below. I plan to use 3 relays.

One set of 2oo3 voted contacts will be wired in parallel with the Master Trip Reset pushbutton, to effect a latch on reset. The reset shall only be effective if all the key-release ESD pushbuttons are healthy.

Another set of 2oo3 voted contacts will effectively become a digital input to the PLC to de-latch the logic.

The 2oo3 relay configuration will permit a maintenance person to depress (on one relay at a time only) the test pushbutton, thereby changing over all the single pole NO contacts on that relay. These NO contacts will each be wired as
digital inputs to the PLC. Therefore, regular testing by maintenance will enable them to verify visually on the HMI if any one of the 2oo3 voted NO contacts is welded in such a way that the contact will not open when demanded to do so.

And finally, the third set of contacts will be used to remove the hot 24vdc from nominated digital output cards.

Essentially, It is prudent I think, to have a mechanism to enable the on-line identification of welding without compromising system availability.
Even if a weld occurs on one relay, the 2oo3 vote will respond to a demand.

Is this over the top? Will I specify individual FMEDA relays or is there a proprietary product to do the above?
It appears that a trip alarm is not the proper choice for this application. However, I do not know of any available relay with FMEDA data. You could check with the three independant firnms that do these analyses: TUV, FM, and "exida.com":http://www.exida.com . They each have websites that list products with certifications.