Open Service Neutral Protection

Mike,

I have several clients that have had significant monetary losses recently due to loss of neutral from the service entrance wiring from the electric utility. This was during a weather situation and the utility claimed that it was an "Act of God" so they won't pay for repairs. I would like to know if there is a commercially available device that can protect the whole house from this condition. Please advise if you know of any such device.

Russell N. Merritt

PS. We are not looking for Transient Voltage Surge Suppression.

Mike Holt's Comment: When the grounded neutral service conductor is opened for one reason or another, the voltage on metal parts (as referenced to the earth) can easily rise to a dangerous value. In addition, the voltage shift between Line 1 and Line 2 can cause destruction of electronic equipment and/or a fire.

Electric Shock

Dangerous touch voltage on metal parts would be present if the grounded neutral service conductor is opened. Since the NEC requires service equipment to be grounded to the earth [250.24(A)], the touch voltage on metal parts, because of an open grounded neutral service conductor will be reduced, but not to a safe value.

The following table demonstrates the touch voltage and current flow through a human having a resistance of 1,000 ohms, as the resistance of the grounding electrode system (GES) [250.50] is reduced if the current on Line 1 is 8A and Line 2 is 3A.

 GES Resistance Touch Voltage* Current Through Human 100 Ohm 49V I = 49V/1000 ohms = 49 mA 50 Ohm 48V I = 48V/1000 ohms = 48 mA 25 Ohm 45V I = 45V/1000 ohms = 45 mA 10 Ohm 38V I = 38V/1000 ohms = 38 mA

Comment: Breathing stops - Often fatal at 40 mA.

* The spreadsheet used for the above values can be downloaded from, www.MikeHolt.com. Click on the 'Free Stuff' link on the left menu; then click on the 'Touch Potential 3-Wire Circuit.'

Equipment Damage and/or Fire

In addition to electric shock, if the grounded neutral conductor is open, the voltage for Line 1 and Line 2 circuits will change. The line voltage for one of the lines will increase and the voltage for the other line will decrease which can result in a fire and/or destruction of electrical equipment.

Example: A 3-wire single-phase 120/240V service supplies a 1,200W, 120V hair dryer and a 600W, 120V television. If the grounded neutral conductor is interrupted, it will cause the 120V television to operate at 160V and consume 1,067W of power (instead of 600W) for only a few seconds before it burns up.

R of the Hair Dryer = 120V2/1,200W
R of the Hair Dryer = 12 ohms
R of the Television = 120V2/600W
R of the Television = 24 ohms

Step 2. Determine the current of the circuit, I = E/R.

I = 240V/36 ohms (12 ohms + 24 ohms)
I = 6.7A

Step 3. Determine the operating voltage for each appliance, E = I x R.

Voltage of Hair Dryer = 6.7A x 12 ohms
Voltage of Hair Dryer = 80V
Voltage of Television = 6.7A x 24 ohms
Voltage of Television = 160V

The following table demonstrates the voltage distribution between Line 1 and Line 2 based on different grounded resistances. A low ground contact resistance helps reduce the difference between Line 1 and Line 2, but not enough to prevent damage to electronic equipment or prevent a fire.

 GES Resistance Line 1 Volts Line 2 Volts 100 Ohm 83V 157V 50 Ohm 86V 154V 25 Ohm 90V 150V 10 Ohm 98V 142V

* The spreadsheet used for the above values can be downloaded from, www.MikeHolt.com. Click on the 'Free Stuff' link on the left menu; then click on the 'Touch Potential 3-Wire Circuit.'

I am not aware of any product that will protect against electric fires or destruction of electrical equipment from an open service neutral.