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PLC Controller Failure Rate

By Don Fitchett

It's not so much what the PLC MTBF (failure rate) is, it is more about what the causes are.

PLC controller failure

As typical with electronics and electrical devices, heat is relative to PLC Controller failure rates.

 

Generally speaking, one should assume a very low probability of failure for PLC controllers. Most of you who are looking for PLC controller failure rate estimates like MTBF statistics, the data is to be an ends to a means. What you are ultimately looking for are answers to questions, like what parts and how many should you stock, what are the weak areas, how to make your equipment more reliable, is a PLC controller more reliable than another form of control, etc. Basically the PLC controller has the life cycle of, and is as reliable as military spec electronics. For marketing and liability reasons, it can be difficult to get PLC controller failure rate estimates from the PLC vendors, but you may convince each of them to point you to that estimated data.

 

Seeking out a PLC vendor's particular PLC MTBF estimate, and planning based on those numbers can be a wasteful effort. The vendor's PLC MTBF is a combination of all modules and all component MTBF. So the PLC MTBF numbers for a PLC controller as a whole will vary by unpredictable amounts given all the various possible module combinations and more importantly given all the variables introduced by the failure factors explained below. Your time would be better spent using the module type reliability list below, and estimate a MTBF for say an output module on one bottleneck machine while taking into consideration your specific failure factors listed below.

 

Example: Even with the highest risk module type, an electro-mechanical relay output module, and all failure factors optimized for reliability, the estimated MTBF, would be of little value. This is because relay failure should be estimated based on activation, not time. One machine may activate a relay on out put card once per hour, and the machine only runs 8 hours a day, 5 days a week. Another machine in same facility, same output card may activate a relay 120 time per hour, and machine runs 24/7. Yet both output cards would have the same MTBF provide by vendor.

 

Because of the lack of recording PLC controller failure rates and causes by end  users, it is impossible to get real world historical data that is not collected. The good news is, as a PLC training company independent of any single PLC vendor, we provide you the real world information you are seeking from the data, in this article. We are able to do this as we have surveyed thousands of plants for over a decade, maintenance and engineers, every industry, around the world, as well as our own very experienced personnel.

 

While keeping in mind there are always exceptions to the rule, PLC controller failure probability will remain very low compared to any other equipment in your facility. PLC controller failure rates will be increased by the following factors (in order of most negative effect on failure rate)...

 

  1. Factor:   Environment
  2. Factor:   Brand/Model
  3. Factor:   Electrical Design supporting PLC
  4. Factor:   PLC Management

 

First of all when considering and preparing for PLC controller failure, it should be on a modular level.  As a PLC controller for the most part is solid-state, which in part is why it is so reliable with very low failure rates.  So when considering failure rates, what parts to stock, reliability improvements, etc.,  you should consider each module type independently …

 

  • PLC processor module
  • PLC power supply
  • PLC communication modules
  • PLC input modules
  • PLC output modules

 

The list of PLC module types just mentioned are in order of most reliable (part with lowest failure rate listed first), with output modules being the most likely to fail. This reliability/failure order is based on the basic equipment failure rate we teach to our PLC training students. Which are ...

 

  • Mechanical devices are more likely to fail than electrical,
  • Electro-mechanical devices are more likely to fail then solid-state devices,
  • High current devices are more likely to fail than low current devices.

 

While considering there are exceptions to every rule, PLC output modules are commonly electro-mechanical (relays) and/or higher current sinking PLC modules. Therefore, they are the first thing you look at for stocking spares and doing reliability improvement analysis on, second is PLC input modules.

 

Now we'll explorer the failure factors for a PLC controller as whole,  the factors mention at the beginning of this artcle. The order starting with most common PLC failure factor ‘Environment’, then ‘Brand/model’, then ‘Electrical Design supporting PLC’, then followed by the one factor you have most control over ‘PLC Management’.

 

PLC Environment: If the electrical panel with PLC controller in it gets exposed to extreme heat or cold, liquids, vibration, dust, etc., the PLC controller failure probability increases.

PLC Brand/Model: While a PLC in general has the lowest failure rate out of all automation control technologies, one brand may be more reliable than another brand. The various models of PLC controllers within any one vendor have a much greater reliability/failure distinction. Example a ‘soft’ PLC installed on a standard consumer PC that cannot handle the rough industrial environment is almost sure to fail. Thus the reason you do not see hardly any of the ‘soft’ PLC models each PLC vendor offers, being used anywhere. As Dick Morley (the father of PLCs) reminded me in one of our phone conversations, “With increased functionality and complicity comes increased failure”. To that point, a PAC model (advanced PLC) will have a higher failure rate than a PLC controller.

Electrical Design supporting PLC: Because the end user of PLC controlled equipment commonly choose the lowest bidder, the equipment designers (OEM) tend to cut corners in design so they can be the lowest bidder. A very common example of this is the combination of the OEM not putting line filters on control side of electrical panel’s transformer secondary and not putting an EEPROM in PLC. Therefore, it is not uncommon during lightning storms and other power noise conditions, that a PLC controller will lose it's program and production goes down.

PLC Management: The best example of this PLC controller failure factor is not having any PLC management or policies in place at all. Also under this category come many failure reduction actions to implement for little or no cost, like electrical PMs (as cleaning electrical panel’s cooling fan filter). Not having an up to date backup of program in PLC controller, not insuring the only personnel working with PLC controllers, have had PLC safety, reliability and best practices training, etc.. Most people working with PLCs today have only received scholastic and theory education, not training.

 

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Categories: plc controller, plc failure rate, plc controller failure, automation controller, equipment failure rate