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Strange voltage from HPFM module

firepuppy

Member
The original owner of this OP2 system had installed a heat tape circuit (for his gutters) and controlled it with an HPFM module connected to a 20Amp single-pole Siemens GPFE breaker.  I measured 68-70 volts when testing this circuit and at first thought it was miswired or that the breaker had failed somehow.  However, when I rewired it with another HPFM module, I observed the following:
  1. When the HPFM was switched ON, I measured 110V on the blue (load) wire.
  2. When the HPFM was switched OFF, I measured 68V on the blue wire (instead of the 0V I was hoping for...).
Both tests were done with the blue load wire disconnected from the circuit.
 
Any thoughts on what my cause such behavior?
 
 

RAL

Senior Member
If you are using a digital multimeter, you are probably measuring a phantom or ghost voltage.   Most digital multimeters have a very high impedance input (10 to 20 megOhms), which creates almost no load on the circuit being measured.  With AC wiring, even when the circuit is unpowered, the wires can pick up a voltage from other nearby wiring through the electric field they generate.  The meter is sensitive enough to measure that.
 
You can verify that this is the case by putting a load across the circuit, such as a light bulb. The heat tape might work too, if there is no thermostat. Or, you can use a digital multimeter that has a Low-Z setting just for this purpose.  An inexpensive analog multimeter with a lower input impedance (usually about 20,000 Ohms/Volt) will also work.
 

ano

Senior Member
firepuppy said:
The original owner of this OP2 system had installed a heat tape circuit (for his gutters) and controlled it with an HPFM module connected to a 20Amp single-pole Siemens GPFE breaker.  I measured 68-70 volts when testing this circuit and at first thought it was miswired or that the breaker had failed somehow.  However, when I rewired it with another HPFM module, I observed the following:
  1. When the HPFM was switched ON, I measured 110V on the blue (load) wire.
  2. When the HPFM was switched OFF, I measured 68V on the blue wire (instead of the 0V I was hoping for...).
Both tests were done with the blue load wire disconnected from the circuit.
 
Any thoughts on what my cause such behavior?
Just for future reference, "Voltage" can't be measured at a single point. Voltage, by definition, is defined as “the difference in electrical potential between two points”. So "110v on a blue wire" doesn't say much if we have no idea where the other wire was. Hot, neutral, hanging in the air?  Were you measuring across a resistive load?  If there is no load, your readings sound perfectly normal.  Was the heat tape not heating?  Most heat tapes have a thermostat that opens above about 30 degrees. 
 

firepuppy

Member
Sorry to be so long in replying; been on the road for a week...
@ano: I should have been more specific about the voltage measurement: with NO load on the circuit (HPFM blue wire disconnected), I measured:
  • 110VAC between the line input (HPFM black wire) and neutral (HPFM white wire)
  • 68VAC between the load output (HPFM blue wire) and neutral (HPFM white wire)   (with the HPFM controller switched OFF)
I had expected to see 0V on the load wire when the controller was OFF.
 
I don't think the issue was stray voltage from nearby wiring (I measured the same 68VAC in the panel (where there were LOTS of other wires generating an electric field) AND outside at the heat tape plug connector (where there were none).
I'm pretty sure I did this same test on other circuits and got a 0VAC drop when the controller was OFF.
I DO think it is a "phantom" voltage, since connecting the load (heat tape) caused the voltage measurement to drop to 0V, AND the heat tape works as advertised when the controller is switched on.
The only explanation I can come up with is that is has something to do with the GFPE (GFI) breaker feeding the HPFM.
 
Anyway, thanks for allaying my concerns about this with your replies.
 
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