Which Standard for design redundant CPU/I/O modules at PLC panel

Dears ALL
I am preparing new PLC panel I want from the contractor to make redundant CPU , power supply and I/O modules, the contractor refuse to make redundant only if there is standard say you have to make redundancy, Please any one help me to get that standard talk when have to make redundant system?

Thanks and best regards

There is no over arching standard that says you must have redundancy. Maybe there's an industry specific standard that mandates it but in my industry (water and waste) we only specify redundancy where the requirement is identified through risk assessment.

If your contractor doesn't want to build what you want... get a new contractor?

Unless you have a contract with them that didn't specify the requirement for redundancy, in which case that's a bit cheeky trying to get it after the fact for the same price (I'd tell you to bugger off as well! )

Saffa said:

There is no over arching standard that says you must have redundancy. Maybe there's an industry specific standard that mandates it but in my industry (water and waste) we only specify redundancy where the requirement is identified through risk assessment.

If your contractor doesn't want to build what you want... get a new contractor?

Unless you have a contract with them that didn't specify the requirement for redundancy, in which case that's a bit cheeky trying to get it after the fact for the same price (I'd tell you to bugger off as well! )

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Many thanks

khlid11 said:

I am preparing new PLC panel I want from the contractor to make redundant CPU , power supply and I/O modules, the contractor refuse to make redundant only if there is standard say you have to make redundancy

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I can understand why you would want a redundant power supply. Can you elaborate on reasons why you want the CPU and I/O to be redundant?

Toine said:

I can understand why you would want a redundant power supply. Can you elaborate on reasons why you want the CPU and I/O to be redundant?

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The control system with redundant is one way to increase reliability and reduce downtime

Rubbish! Carry a spare processor and rack in case of a failure. I use Omron and apart from getting drowned with chlorine, lightning strikes and cement + water = sulfuric acid I have not had a failure in over 12 years.

khlid11 said:

The control system with redundant is one way to increase reliability and reduce downtime

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I would say that this can be a tad bit misguided as a whole.

The CPU unit as long as the power supply is good and not prone to spikes lasts an incredible amount of time.

I would, and usually do, pay a lot more attention to redundancy on the supply to the panel (or panels) because that is much more likely to fail, redundancy on the network such as making a ring, etc... and redundant instruments making use of distributed IO.

These do make a big impact on the redundancy and uptime of the equipment, the CPU itself in my experience is only done for when things can't go wrong in any way shape or form such as offshore drilling equipment, turbines, etc...

Either way, a risk assessment is the way to go since it highlights the risks first and you can then put measures in place for each of them.

There are certainly instances where hot standby is a requirement, and simply carrying a spare is not sufficient.

We operate a major wastewater pumping station where should the controls fail, the station overflows into a marine environment within 2-6 minutes depending on inflow. $600k fine if that happens.

The original design had two 3000A switchboards fed by independent transformers, and an automatic start generator that could supply either bus. 5 x 250kW pumps on each. However, only a single PLC with a common modbus plus bus to terminal IO in each MCC.

We recently upgraded to a Schneider M580 Hot Standby after a failure on one connector on that cable brought down the whole bus and we narrowly escaped that fine. We now have a redundant Ethernet ring for the IO, dual racks with redundant instruments distributed accordingly (i.e. sensor A on Rack 1 sensor B on Rack 2). Redundant DC supplies, redundant rack power supplies etc.

We can turn off a whole CPU rack by killing the power and the standby unit takes over so quickly nothing even stops operating.

khlid11 said:

The control system with redundant is one way to increase reliability and reduce downtime

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Only after a risk analysis shows that this is required and will actually increase reliability. The case as illustrated by Saffa is a good example where it does pay off. Keep in mind the whole design of the system needs to be quite different and becomes more complicated. If not done properly but rather just dual everything for the sake of redundancy then you have just made your panel three times the original cost, way more difficult to understand let alone troubleshoot and it may have introduced more points of failure. Unless done properly you are spending more money for a less reliable system. Without further explanation of the reasons behind your question my initial guess is still: keep it simple. PLC's are rather robust in a properly designed system.

Toine said:

Keep in mind the whole design of the system needs to be quite different and becomes more complicated. If not done properly but rather just dual everything for the sake of redundancy then you have just made your panel three times the original cost, way more difficult to understand let alone troubleshoot and it may have introduced more points of failure. Unless done properly you are spending more money for a less reliable system.

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This is the important bit... it's never a case of just doubling up. And the cost is always higher than twice the price (and that's not accounting engineering hours)... sadly, the "Engineering" Manager at my last job didn't quite understood any of these concepts and wanted redundancy without paying for it.

Many Thanks