Today's electricians are often found working in applications outside the realm of what is traditionally considered "electrical." Through new programs sponsored by the IBEW-JATC training schools and others, the skill sets of the electrical union workforce have broadened to include automation and controls.
Nowadays many industrial electricians are just as comfortable troubleshooting a circuit breaker panel as a PLC (Programmable Logic Controller). It is becoming more common for electricians to work in automation applications, where they frequently troubleshoot the inputs and outputs (I/O) of control systems.
Commercial electricians, meanwhile, are rapidly training on building controls, as retrofits and retro-commissioning become larger opportunities. New efficiency upgrades double or triple the demand for low voltage and milliamp troubleshooting.
In 2007, the first clamp meter to accurately measure milliamps without breaking the circuit was introduced. It could simply clamp on and read down to 0 milliamps and up to 100 milliamps. The tool saved so much time that it revolutionized automation and controls troubleshooting. Interestingly, electricians adopted the tool in even greater numbers than industrial technicians.
Now, new versions of this clamp meter have been engineered with controls electricians specifically in mind. By incorporating the functions of a loop calibrator, including low voltage measurement and sourcing, the meters will save electricians even more time and vastly increase their troubleshooting capabilities in both automation and controls.
Measurement Capabilities
Where the original clamp meter measured milliamp signals, one new model adds milliamp sourcing, simulation and 24 V loop power (for testing power supplies to control loops). Another model further adds voltage sourcing and measurements (for low voltage building controls), simultaneous mA sourcing and measurement (for testing I/O) and mA output, to log over time and capture intermittent problems.
Automation Overview
Programmable Logic Controllers (PLCs) are the control system of choice for many industrial applications, ranging from water treatment facilities to high speed automation applications like bottling lines. Most PLC inputs and outputs are "ones and zeros" representing either the status of devices, such as a switch or sensor, or outputting a signal to close a switch or relay, open or close a valve or control a motor in a process.
Many inputs and outputs are analog and represent a measured variable such as temperature, pressure or to what degree a valve is opened or closed. These measured variables are represented as inputs to the PLC using the 4 to 20 mA signal. A good example would be where the control system needed to know the position of a valve (percent open or closed). If the output signal from a normally closed valve is 4 mA, the control system interprets this as the valve being closed. If the valve output signal is 20 mA the control system interprets this as the valve being open. When the electrician is troubleshooting analog I/O, measuring and interpreting the 4 to 20 mA signal is essential.
Building Controls Overview
While control systems are often a jumble of digital, analog and pneumatic controls, the analog component reliably uses either low voltage or milliamp loops, or sometimes both. The purpose is similar to the automation description above-these loops transmit information and instructions from sensors and devices to controllers upstream.
Sensors communicate by converting their output signal to a 4-20 mA dc current, with 4 mA representing the sensor's zero-level output and 20 mA representing the sensor's full-scale output. Other systems use 1-5 V dc or 0-10 V dc circuits in a similar fashion. Occasionally, the 4-20 mA signal is converted to a 2-10 V dc signal by adding a 500 ohm resistor across the load.
Most control systems use fully-electronic, two-way, three-way and Pressure Independently Characterized Control Valves (PICCVs).Variable frequency drives (VFDs) are used as standard equipment on towers, variable air volume (VAV) fans, pumps and chillers.
Typical sensors include humidity and temperature transmitters, CO2 sensors for indoor air quality, power meters, branch circuit monitors and energy meters. Field level controllers talk to sensors and actuators, often using 4-20 mA or 0-10 V dc analog signals.
Troubleshooting 4 to 20 mA Input Output Circuits
Often the first indication of a control loop problem comes from the operator or manufacturing manager: "I think we have a bad valve," or "This control system is not responding the way it used to." In either case, it is the electrician's signal to begin troubleshooting.
The first step is to measure the 4 to 20 mA signal, either by breaking the loop connecting in series with a DMM, or by using a mA clamp meter and verifying the loop current value. If the loop current measured is not as expected, there are three likely causes: broken/disconnected/shorted wires, a bad loop power supply or faulty instrumentation.
If no problem is found in the wires, use a DMM (or the voltage measurement function on specific clamp meters) to check the loop power supply. If the power supply shows no output, use the 24 V loop power function of the clamp meter to substitute for it; if the loop then works properly the source of the problem is obvious.
If the wiring and the power supply both check out, it is time to check the transmitter or other similar instrument in the control loop. With a loop calibrator, process calibrator or multi-function clamp-on meter, use its mA simulate mode to substitute for the transmitter. If the loop performs as requested, the problem lies with the transmitter; if not, it is elsewhere.
If a final control element (valve positioner, etc.) is suspected, use the mA source/simulate mode on the clamp meter to feed a signal into it while watching the local indicator for a response.
Loop Malfunctions
If the problem is not a dead loop but an inaccurate one, likely possibilities include a bad I/O card on the PLC or
