Raychem Industrial Heating System FAQ (part 2)

 

Test and Inspection of Self-Regulating and Power-Limiting Heating System Cables

 

 

Visual Inspection

 

• Check inside  heating cable  components for proper installation, overheating, corrosion, moisture, and loose  connections.
• Check the electrical connections to ensure that ground and bus wires  are  insulated over their  full length.
• Check for damaged or wet thermal insulation; damaged, missing or cracked lagging and weather-proofing.
• Check that  end seals, splices, and tees are  properly labeled on insulation cladding.
• Check control and monitoring system for moisture, corrosion, set  point, switch  operation and capillary damage.

 

 

 

Insulation Resistance (Megger) Test

 

Frequency

Insulation resistance testing is recommended at five stages during  the installation process and as part  of regularly scheduled maintenance.

• Before  installing the cable
• Before  installing components
• Before  installing the thermal insulation
• After installing the thermal insulation
• Prior  to initial start-up (commissioning)
• As part  of the regular system inspection
• After any maintenance or repair work

 

Procedure

Insulation resistance testing (using a megohmmeter) should be conducted at three voltages; 500, 1000, and 2500 Vdc. Significant problems may not be detected if testing is done only at 500 and 1000 volts.

 

First measure the resistance between the heating cable  bus wires  and the braid (Test A) then  measure the insulation resistance between the braid and the metal pipe (Test B). Do not allow test  leads to touch junction  box, which can cause inaccurate readings.

• De-energize the circuit.
• Disconnect the thermostat or controller if installed.
• Disconnect bus wires from terminal block, if installed.
• Set test  voltage  at 0 Vdc.
• Connect the negative (–) lead to the heating cable metallic braid.
• Connect the positive (+) lead to both heating cable bus wires  simultaneously.
• Turn on the megohmmeter and set  the voltage to 500 Vdc; apply the voltage  for 1 minute. The meter needle should stop moving. Rapid deflec- tion indicates a short. Record  the insulation resistance value in the Inspection Record.
• Repeat Steps 4–7 at 1000 and 2500 Vdc.
• Turn off the megohmmeter.
• If the megohmmeter does not self-discharge, discharge phase connection to ground with a suitable grounding rod. Disconnect the megohmmeter.
• Repeat this test  between braid and pipe.
• Reconnect bus wires  to terminal block.
• Reconnect the thermostat.

 

Important: System  checkout and regular  main- tenance procedures require  that insulation  resistance testing be performed from the distribution panel unless a control and monitoring system is in use.  If no control system is being used,  remove  both power feed wires from the breaker  and proceed  as if testing heat- ing cable bus wires.  If a control and monitoring system is being used,  remove  the control equipment  from the circuit and conduct the test directly from the heating cable.

 

WARNING: Fire hazard in hazardous  locations. The insulation  resistance test can produce sparks. Be sure there  are no flammable vapors in the area before  performing  this test.

 

Insulation Resistance Criteria

A clean, dry, properly installed circuit  should mea- sure thousands of megohms, regardless of the heat- ing cable  length or measuring voltage  (0–2500 Vdc). The following criteria are  provided  to assist in deter- mining the acceptability of an installation where optimum conditions may not apply.

 

All insulation resistance values  should be greater than  1000 megohms. If the reading is lower, consult Section  10, Troubleshooting Guide.

 

Important: Insulation resistance values  for Test A and B; for any particular circuit, should not vary more than 25 percent  as a function of measuring voltage. Greater variances may indicate a problem with your heat-tracing system; confirm proper installation and/ or contact Pentair Thermal Management  for assis- tance.

 

 

 

Power Check

 

The power  output of Self-Regulating and Power- Limiting cable  is temperature-sensitive and requires the following special procedure to determine its value.

• Power  the heating cable  and allow it to stabilize for 10 minutes, then  measure current and voltage at the junction box. If a thermostat or controller is used, refer to details below.
• Check the pipe temperature under the thermal insulation at several locations.
• Calculate the power (watts/ft) of the heating cable by multiplying the current by the input voltage and dividing by the actual circuit  length.

 

Ambient-sensing controlled systems

If the actual ambient temperature is higher than  the desired thermostat setting, turn  the thermostat set- ting up high enough to turn  on the system, or (with some models) manually set  the selector switch  to the ON position.

• Turn on the main circuit  breaker.
• Turn on the branch circuit  breakers.
• After a minimum of ten minutes, measure the volt- age, amperage, ambient temperature, and pipe temperature for each  circuit  and record the values in the “Installation and Inspection Record”  (refer  to Section  11). This information is needed for future maintenance and troubleshooting.
• When the system is completely checked out, reset the thermostat to the proper temperature

 

Line-sensing controlled systems

Set the thermostat to the desired control tempera- ture,  or to a setting high enough to turn  the circuit on if the pipe temperature is above the control temperature.

• Turn on the main circuit  breaker.
• Turn on the branch circuit  breakers.
• Allow the system to reach the control point. This may take  up to four hours for most  circuits. Large, liquid-filled pipes  may take  longer.
• Measure the voltage,  amperage, and pipe tem- perature for each  circuit  and record the values  in the “Installation and Inspection Record”  (refer  to Section  11). This information is needed for future maintenance and troubleshooting.
• When the system is completely checked out, reset the thermostat to the proper temperature.

 

Control and monitoring systems

Refer to the installation instructions supplied with the product for commissioning tests and records.

 

 

 

Fault Location Tests

 

Fault location

There are three methods used for finding a fault within a section of heating cable: the ratio method, 1/R method, and the capacitance method. The capacitance method can also be used to determine total heating cable length.

Ratio Test Method

a.) To locate  bus wire short:

The ratio method uses resistance measurements taken at each  end of the heating cable  to approxi- mate the location  of a bus wire short. A shorted heating cable  could result in a tripped circuit  break colder or a Braid section of pipe.

 

Measure the bus-to-bus conductor resistance from the front end (measurement A) and the back end (mea- surement B) of the suspected section.

The approximate location  of the bus wire short, expressed as a percentage of the heating cable length from the front end, is:

The fault is located 40% along  the circuit  as measured from the front end (A).

 

b.) To locate  low resistance ground fault:

 

To locate a low resistance ground fault, measure resistance between bus and braid.

The approximate location  of the fault, expressed as a percentage of the heating cable  length from the front end (A), is:

The fault is located 40% along  the circuit  as measured from the front end (A).

 

c.) To locate  severed section:

This method uses the core  resistance of the heating cable  to approximate the location  of a fault when the heating cable  has  been  severed and the bus wires have not been  shorted together. A severed cable  may result in a cold section of pipe and many not trip the circuit  breaker.

Measure the bus-to-bus heating cable resistance from the front end (measurement A) and the back end (measurement B) of the suspect section.

 

The approximate location  of the fault, exprBessed as a percentage of the heating cable  length from the front end (A) is:

The fault is located 20% from the front end (A) of the circuit.

 

Capacitance Test Method

 

This method uses capacitance measurement (nF) to approximate the location  of a fault where the heating cable  has  been  severed. It also gives an estimate  of total  heating cable  length in a non-severed circuit. This reading must be taken at the power  connection and will only work when the heating cable  has passed IR testing. This information is used  to calculate the heating cable  output per linear  foot or to determine if the maximum length has  been exceeded.

 

Record  the capacitance reading from one end of the heating cable.  The capacitance reading should be measured between both bus wires  twisted together (positive lead) and the braid (negative  lead).

 

Multiply the measured capacitance with the heating cable’s capacitance factor  as listed  in the following table.

 

Example:

      20XTV2-CT

      Recorded capacitance = 16.2 nF

      Capacitance factor = 10.1 ft/nF

      Fault  location = 16.2 x 10.1 nF

= 164 ft (50 m)

                                                   from reading location

 

As an alternative, capacitance values  from both the front and back end can be used. The ratio of one capacitance value taken from one end (A) divided by the sum  of both A and B (A + B) and then  multi- plied by 100 yields the distance from the first end, expressed as a percentage of the heating circuit length.