Cold weather is upon us again, and with it comes the annual hope that all is well with your plant’s electric heat tracing system. While these systems are robust in design and manufacture, most process engineers have seen issues crop up with the onset of cold weather. Here is how to address some of the problems typically encountered at this time of year and provide a guide to determine root causes and corrective actions. Common issues include:
- Circuits tripping circuit breakers.
- Circuit temperatures that are too low.
- Circuit temperatures that are too high.
While this article addresses commonly encountered issues, it is not intended to be an exhaustive list or to replace service from your heat trace supplier. However, it can be used to ensure that you cover the basics and understand what your heat trace service provider should look for to address your particular system’s problems. Also, as with all industrial electrical work, heat trace system service work should only be performed by trained, qualified and, in areas required by law, licensed heat trace service technicians. This is critical to ensure the safety of plant personnel and the protection of plant equipment. Any and all test equipment used should be calibrated and in good working condition. All plant safety rules and guidelines for lock-out tag-out, hot work permitting and the like should be adhered to without fail.
Industrial Heat Tracing Circuits Tripping Circuit Breakers
The most often encountered system problem is a circuit breaker tripping. This fault can be broken down to two categories. The first is when the circuit trips instantaneously on power up. The second is when the circuit trips after a few seconds of operation. It is critical to observe the trip time as this is a clue to begin the troubleshooting process.
Circuit Trips on Power Up. If the problem is that the circuit trips on power up, it is likely a short circuit to the ground in the system. Areas of concern for this issue include:
- The cable itself.
- The power connection.
- The power wiring.
Any act of troubleshooting should include a megger check. A megger can be used to test the insulation resistance of both the cable and power wires to isolate potential faults to ground. The megger applies a voltage (usually 500 or 1,000 VDC) between the metal braid or ground, and the cable conductor. (In the case of the heating cable, the cable conductor is the semiconductive core; in the case of the power wiring, the cable conductor is the copper wire.) The suggested method is to isolate the heating cable from the power wiring in the cable power connection box and test the heating cable between the braid and buss wire first. Ensure that the insulation resistance meets or exceeds the manufacturer’s recommended minimum.
If the insulation resistance does not meet or exceed the manufacturer’s recommended minimum, there are three areas to check.
First, be sure that the cable conductive core material is not in contact with the metal grounding braid or other metal parts of junction box. Check all power connections, splices and tee boxes, and end seals to ensure that the conductive core material is isolated from all metal parts, including the grounding braid. If the conductive core contacts metal, make the necessary repairs and run a megger check again until the insulation resistance is above manufacturer’s minimum requirements. While checking junction boxes and end seals, be sure that all connections are dry. Wet junction boxes or end seals can lead to megger test failure.
Second, if no metal parts or braiding are found to be in contact with the conductive core, and the insulation resistance is still too low, isolate each piece of heating cable on the circuit and run a megger check independently. This condition may identify an area of physical damage to the heating cable that is causing the conductive core to short to the metal grounding braid or a pipe. Inspect the piping system and look for obvious signs of damage on the sections that test low. If there are no obvious signs of damage, it may best to remove and replace the section of cable that is testing low. Longer sections can be segmented and tested separately to isolate the fault. Once the fault is isolated, remove the offending section of cable and replace it with new cable. Conduct a megger test on the new installation to ensure that the new cable is good.
Third, if the heating cable sections test good, then re-run the megger check on power wiring. The short could be in the power wiring that runs from the cable junction box back to the heat-trace panel. If the power wiring has a short, remove it and replace with new wire.
Circuit Trips After Startup. Another common problem is when the circuit trips after a few seconds of operation. This condition usually indicates a problem due to the startup current generated by self-regulating heating cables. Possible causes include:
- The startup temperature is lower than the designed startup temperature.
- The installed circuit length is too long for the breaker size.
- The ground fault current trip level may be adjustable and set too low.
- The circuit breaker may not have the correct trip curve for self-regulating cables.
Solutions for these possibilities include checking the circuit breaker rating and the manufacturer’s maximum circuit length recommendation for your startup temperature vs. the installed length. In many cases, the cable circuits are designed to be started at a set temperature for freeze protection -- for example, at 40oF (4oC). If the cable is run to the maximum circuit length for a 40oF startup temperature, and the ambient temperature is actually lower, the breaker will nuisance trip until the cable’s conductive core is warm enough to draw acceptable current to the breaker. This can be solved temporarily by cycling the circuit breaker on and off until the cable warms up. If the circuit length is in excess of the maximum length for the manufacturer’s start temperature and breaker size recommendation, then the circuit length must be reduced. This can be handled by making two or more circuits that fall within the manufacturer’s recommended lengths.
If breaker sizing and startup temperatures conform to manufacturer’s specifications, then check to be sure the installed cable length is within the maximum circuit length limits. If the cable length is too long, reduce the circuit length by creating the correct number of circuits for run length, breaker size and startup temperature.
If the heat trace panel has adjustable ground-fault-current detection, be sure the minimum setting is 30 mA. Settings below this threshold may induce nuisance tripping on longer circuit runs. Be sure that the ultimate setting is within plant safety guidelines and conforms to local codes. If personnel-protection ground-fault detection is required, do not adjust the trip level above 4 to 6 mA. If adjusting the ground-fault trip-level does not solve the problem, run a megger check to be sure the cable’s conductive core is not shorted to metal, pipe or braiding. If the megger check is within reason, then reducing overall circuit length is the next best solution.
You also may check that the circuit breakers used are thermal magnetic type. These breakers have a trip curve that is somewhat more forgiving than a standard breaker. Most heating cable manufacturers have determined their maximum circuit lengths using trip curves for a thermal-magnetic breaker as the model for handling startup current.
Industrial Heat Tracing Circuit Temperatures Are Too Low
Possible causes for issues related to low circuit temperatures include:
- A thermostat or process controller setpoint that is not correct.
- A thermostat that is not wired correctly.
- A cable that is not connected to power.
- A cable that is connected to wrong voltage.
- A temperature sensor that is incorrectly placed.
- A temperature sensor that is wired incorrectly.
- The amount of cable used is insufficient to offset the heat sinks in the system.
To solve these problems, take a systematic approach. First, ensure that your thermostat or process control is set at the desired pipe temperature. Second, check that the thermostat is wired to close upon setpoint. Most thermostats have the ability to be wired from a common terminal to either a normally open or normally closed position. Check that the thermostat is wired from the common terminal to the normally closed position for freeze protection applications.
Third, double-check that the cable is connected to the power. Test the power in both the cable power connection box and in the cable end seal. Testing at the end of circuit also will ensure that the heating cable has two good buss wires down the entire length of the cable. The voltage readings at the start and end of each circuit should be relatively similar. Of course, some voltage drop will occur over a long run of cable, and the final number varies from cable type and manufacturer, so check the documentation. However, if testing shows 120 V at the start of a cable run and zero at the end, you know that at least one of the cable buss wires is damaged, and the cable needs to be removed and replaced.
Fourth, it is important to be sure that the cable is connected to correct voltage. When doing so, compare measured values to design documents. Remember, 240 V cables powered at 120 V will not maintain the correct pipe temperature. Make adjustments as necessary to correct voltage issues.
Fifth, ensure that the temperature-sensing device is located in an area representative of the coldest pipe temperature to be encountered. Ambient sensors should be located away from heat sources such as sunny areas and steam traps; ideally, they should be located on the coldest, most exposed part of the building. This will ensure that the cable operates when needed. Line sensors should be located at least 90o (on the pipe) away from the heating cable so pipe temperature -- not cable-sheath temperature -- is measured. These sensors also should be located away from larger heat sinks and placed at the coldest expected end of the heat trace line. Placing a sensor at the outlet of a hot tank, for a heat trace system that is designed to maintain fluid temperature over a long run of cable to a holding tank or distribution station at a lower temperature, is formula for disaster. In this case, the fluid enters the piping system at or above the desired temperature. The temperature sensor does not see temperatures below the setpoint, so the cable system is not energized. And, in the meantime, the fluid cools over the length of the run and plugs the line at the holding tank or distribution station. In this case, the sensor should be located as close to the holding tank or distribution station as possible.
Sixth, make sure the temperature sensors are wired per the manufacturer’s instructions. It is easy to wire a three- or four-wire system incorrectly and end up turning your system off at a temperature that should call for heat. While this happens all too often, it is very easy to fix.
Finally, be sure all larger heat sinks -- valves, pumps, vias through walls and other obstacles -- have sufficient cable to maintain pipe temperature. Follow the manufacturer’s recommendations regarding any required extra cable at these heat sinks as well as for extra cable at pipe shoes and supports.
Industrial Heat Tracing Circuit Temperatures That Are Too High
When too high circuit temperatures are causing problems in your heat tracing systems, consider these possible causes:
- Incorrect setpoint on the thermostat or process controller.
- Incorrect location of the temperature sensor.
- Incorrect wiring of the temperature sensor.
- A faulty thermostat.
Troubleshooting steps for these possible problems include checking to be sure that the thermostat or process controller setpoint is set to the desired pipe temperature. Also, check whether the temperature sensor is in the proper location.
In addition, for process applications, be sure that independent temperature sensors are used for each pipe size and flow path. Attempting to use the same sensor for multiple pipe diameters and flow paths can lead to overheating pipes with smaller diameters or low flow, depending on sensor placement. Temperature sensors also must be wired according to the manufacturer’s instructions to ensure proper operation.
It also is wise to check the thermostat to be sure it has not been subjected to excessive heat or electrical current. These conditions can cause permanent contact closure, which causes the system to call for heat regardless of thermostat setpoint. If thermostat is found to be faulty, it should be removed and replaced.
Following these troubleshooting steps should solve many common heat tracing system issues. While this article is not intended to solve every situation that arises with an electric heat trace system, it should save you some time and trouble during the heating season.