Pilot Cable Monitor Function
The PCM177 pilot cable monitor is an earth continuity relay. It provides earth continuity protection in accordance with AS/NZS 2081 ensuring the earth connection through a trailing/reeling cable is maintained. A pilot cable is connected to earth via a diode, the PCM177 detects open and short circuits between the pilot and earth conductors of the trailing cables.
Find a Product Function = PCM
What is a Pilot Cable?
The pilot cable is an additional cable in a cable used to detect if the earth core of the cable is intact.
APCS does not recommend any particular cable configuration or brand.
Typical Site Layout
The PCM177 should be installed in a Supply Enclosure located where the electrical power is distributed to the remote equipment and includes electrical switching to isolate the power from the Remote Enclosure.
The supply enclosure should have a solid earth connection point. The PCM177 is wired to earth, while the remote equipment is earthed through the trailing/reel cable.
The trailing/reel cable runs between the supply enclosure to the remote enclosure. This cable needs to comply with appropriate standards applicable to the region where the equipment is installed. Most approved cables will have an outer screen which should be connected to earth at the supply end only. The cable will generally include three phases, 1 to 3 earth cores and a central Pilot Core.
.
Typical Site Layout
How Does PCM177 Work?
Typical PCM177 Waveform
Measurement between terminals 9 -ve and 10 positive.
The PCM177 presents a 210Hz bipolar square wave signal on the pilot core of the cable and the earth core(s).
A positive signal constant current (28.58mA for Automatic reset, 21.76mA for remote start) this positive current passes through the remote diode to determine the pilot cable resistance.
The negative constant current of 3.8mA is blocked by a diode which enables the measurement that represents a short or partial short between the pilot cable and earth which occurs at 600R.
The wave form displayed was produced with the test circuit below. As the circuit has no cable leakage resistance fitted before the diode (between 9 and 10 ) the negative side of the drive goes to the rail limit of -14V.
Cable Noise Immunity
As seen above the sensing circuit has a +/-15V measurement range and the cable resistance has a 20mA drive. We are limited in the drive current/voltage to remove any risk of producing a spark.
Induced noise will affect operation of the unit if the noise current is greater than 5mA which is not an insignificant figure.
We injected a 50Hz signal into the pilot measurement cable using a transformer connected in series. The transformer introduced an additional 5.7ohm resistance into the pilot loop.
If you have a noise problem
Check the waveform of the signal on terminals 9 and 10 of the PCM177 and send us a picture for possible advice.
Make sure any outer screen on the cable is only connected at one end.
Make sure that there is not a problem with the load equipment.
Check that the centre core is used as the pilot cable.
Use a PCM177 model that does not automatically reset from the fail condition.
Pilot Cable Noise Immunity Test
Cable Noise Immunity with 45 Ohm non latching : PCM177-14400 When RS (the simulated pilot resistance) was at 63 ohm and the injection noise measured V2 was 10V,the relay would switch between healthy and not healthy.
| Test Result / Condition | RS ohm | V1 rms | V2 Vrms |
|---|---|---|---|
| Alarm coil bypassed | 48.00 | 8.05 | 0.65 |
| Healthy coil bypass | 45.00 | 8.02 | 0.61 |
| Alarm with min variac | 43.00 | 8.46 | 3.15 |
| Healthy with min variac | 40.00 | 8.44 | 3.18 |
| Alarm: 5Vac on V2 | 42.00 | 8.85 | 5.00 |
| Healthy: 5Vac on V2 | 40.00 | 8.83 | 5.01 |
| Alarm: 7Vac on V2 | 40.00 | 9.47 | 7.02 |
| Healthy:7Vac on V2 | 39.00 | 9.46 | 7.02 |
| Alarm: 10Vac on V2 | 63.00 | 10.77 | 10.00 |
| Healthy: 10Vac V2 | 61.00 | 10.76 | 10.01 |
Noise Injection Test Circuit