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Maintenance
Insulation Resistance Testing, Part 5
Sometimes, an insulation resistance (IR) testing program is in place, yet a major cable failure still occurs. Because it doesn't seem to work, cutting back or eliminating the program appears to make sense — that is, until you look deeper.

In these situations, what hasn't happened is consistent testing followed by consistent trending. What makes an IR program work is the ability to trend data over time. A proper trend can't exist unless you do the same tests the same way on the same schedule every time. The only variation in the testing should be temperature adjustment, which also must be done in a consistent manner.

Figure out ahead of time what the testing interval will be for a given type of equipment, and stick to that interval. If you don't, then the program will fail as will your equipment. To determine that interval, you must balance a realistic level of access (and required downtime) against the minimum testing frequency that will allow you to detect insulation deterioration in a timely manner.

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The Ins and Outs of Ladder Diagrams
In a PLC-controlled system, problems rarely occur inside the PLC. A malfunction is nearly always on the input side or output side of the PLC. The ladder diagram shows you what the inputs and outputs are, and it shows how each output responds to a given input change. This is, in fact, what makes a ladder diagram especially useful for troubleshooting — you can follow the current flows of control circuits to find what's wrong.

The output of one control circuit is often the input to another elsewhere on the drawing. For example, at line 27 on your drawing, a limit switch is an input to a relay control circuit. The circuit ends with a coil contact (output). When tripped, the switch provides a signal and the coil picks up, thereby energizing the contact.

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Electrical Troubleshooting Quiz
Let's use that mixer example from above. Suppose you can see that the limit switch is tripped, but there's no power coming out of the motor starter. Can you outline the next few troubleshooting steps?

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Operation
NEC in the Facility
The typical maintenance department has several spools of THHN wire on a rack or pallet, ready for projects or repairs. THHN is a good, general-purpose insulation. However, suppose you need to rewire a machine tool. Would THHN be the most appropriate kind of wire? Would it be appropriate at all?

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Protect Your Control System from Potential Cyber Attacks
Like most facilities, yours probably has established maintenance, availability, and safety programs in place. But is there a program that guards your control system against possible cyber attacks?

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