240V Circuit Problem

[davidgarrett]

I have a customer who asked me to install two sets of baseboard heaters yesterday, two heaters on each set. There are two 240V circuits in place and ready to use. One circuit is fine, the other is not. On the one that is not, I measured 120V to ground on each leg, but 0V between the two legs at the end of the circuit. At the breaker, I measured 120V on one leg and 35V on the other leg. The circuit is run between floors so these are the only areas where the wire is accessible. These are replacing old baseboard heaters the customer removed a few months ago. The only other info I have from the customer is that heaters are being replaced because one of them, the one on the above mentioned circuit, caught fire some time ago. Anyone know what the problem is? Thanks!

 

[kwired]

You likely have a lost line in a feeder or service, blown fuse, or one side of a breaker is not closing.

Are there other 240 volt loads and if so are they doing the same thing? What about the supply to the panel?

 

[gar]

130427-1023 EDT

davidgarrett:

I assume you are using a high input impedance meter.

At the main panel at the input side of the main breaker or fuses measure each hot line to neutral, and line to line voltage. Are these nominal values of 120, and 240?

Assuming those input voltages are as expected, then measure these same voltages at the output, but on the terminals, of the two-pole breaker that supplies the problem circuit. what are these values?

If the values are as expected, then do the same measurement but directly on the conductor leaving the breaker.

If the output of the breaker is not what you expect, then you have a breaker problem.

With a high impedance meter a voltage like 35 V is probably from capacitive coupling from an adjacent wire. Put a 120 V incandescent 100 W bulb as a load on that wire and the voltage will probably drop to near 0.

If a 240 V heater was connected only to a 240 V source, and no connections to ground or neutral existed, and there was no sustained over voltage, then that heater did not burn out as a result of the wiring or circuit breaker. There would be nothing greater than 240 in the house under normal circumstances.

.

 

[mjf]

Do the wires go directly to where the heater will be mounted or is there a 240v thermostat in the equation?

 

[davidgarrett]

130427-1023 EDT

davidgarrett:

I assume you are using a high input impedance meter. Yes.

At the main panel at the input side of the main breaker or fuses measure each hot line to neutral, and line to line voltage. Are these nominal values of 120, and 240? Yes.

Assuming those input voltages are as expected, then measure these same voltages at the output, but on the terminals, of the two-pole breaker that supplies the problem circuit. what are these values? 120V and 240V

If the values are as expected, then do the same measurement but directly on the conductor leaving the breaker.

If the output of the breaker is not what you expect, then you have a breaker problem.

With a high impedance meter a voltage like 35 V is probably from capacitive coupling from an adjacent wire. Put a 120 V incandescent 100 W bulb as a load on that wire and the voltage will probably drop to near 0. I'll try to put a load on it when I go back next week.

If a 240 V heater was connected only to a 240 V source, and no connections to ground or neutral existed, and there was no sustained over voltage, then that heater did not burn out as a result of the wiring or circuit breaker. There would be nothing greater than 240 in the house under normal circumstances.

.

Can you help me understand capacitive coupling?

 

[ronaldrc]

Has any one been working on those circuits in the panel and maybe have put your bad one on a Tamdem breaker.

Your voltage measurementd sound just like one 240 volt circuit is being fed from the same line.

Ronald

 

[gar]

130427-1429 EDT

davidgarrett:

Two conductors separated by an insulator form a capacitor. The capacitance is a function of the geometry of the conductors, spacing, and the dielectric constant of the insulator.

For a two conductor piece of #12 Romex with an uninsulated ground wire between the two insulated wires the capacitance from one wire to the ground wire is about 24 pfd per foot, and between the two insulated wires with the ground conductor unconnected to anything the capacitance is about 15 pfd per foot.

Using a Fluke 27, input impedance 10 megohms shunt by a small capacitor, a 5.5 ft sample of Romex and 123 V 60 Hz I measured 39 V using the ground conductor and one outside conductor as the capacitor, and 26 V when using the two outside conductors. These values are in the ballpark of what can be expected based on the capacitive reactances of the two different connections.

.

 

[kwired]

130427-1429 EDT

davidgarrett:

Two conductors separated by an insulator form a capacitor. The capacitance is a function of the geometry of the conductors, spacing, and the dielectric constant of the insulator.

For a two conductor piece of #12 Romex with an uninsulated ground wire between the two insulated wires the capacitance from one wire to the ground wire is about 24 pfd per foot, and between the two insulated wires with the ground conductor unconnected to anything the capacitance is about 15 pfd per foot.

Using a Fluke 27, input impedance 10 megohms shunt by a small capacitor, a 5.5 ft sample of Romex and 123 V 60 Hz I measured 39 V using the ground conductor and one outside conductor as the capacitor, and 26 V when using the two outside conductors. These values are in the ballpark of what can be expected based on the capacitive reactances of the two different connections.

.

Now lets add that this capacitor is very weak and if you put a small load across it, it is not strong enough source to sustain the load and the voltage totally sinks to nothing. High impedance meters will read this voltage but a low impedance meter lets enough load thru to short this capacitor.