Here is an extremely low-cost way to stop ground-faults from burning out electronics, refrigerator, microwave, etc, in your RV
A ground fault
occurs when the neutral side of the RV power cord become open - due to an
intermittent or break in your:
50A Power cable connecting the RV to the park outlet
RV park outlet
RV park distribution system
Over-voltage protection system - typically put between your cable and the RV park outlet
Wiring in your RV (least likely condition)
Only 50A connections have the possibility of 240 volts when there is an
A 30A connection cannot burn out electronics, as it very rarely has more than 115 VAC.
Solution - exploit the ground fault protection
Ground Fault Protection, also known as Ground Fault Circuit Interruption or GFCI has been required by law for all outlets near water in homes and RVs for over 30 years
Just install a 1,000 ohm 1/10 watt resistor in an AC plug
Buy the resistor at Radio Shack and screw it into an AC plug between the neutral and ground
It will protect all of the equipment installed downstream from the GFCI device - which is typically your refrigerator and some of your electronics
You will need GFCI circuits and plugs with resistors to protect again
open-neutral on other circuits
- such as microwave, battery charger, home entertainment.
GFCI devices are typically outlets which replace existing outlets - cost less than
Replacing the first outlet in a circuit with a GFCI outlet protects only the rest of the electrical equipment on that circuit, not the rest of the RV
You might consider other GFCI adapters on additional circuits
- GFCI apapter plugs into a wall outlet - either directly or as a short extension cord - costs $15 and up
- GFCI circuit breaker - which replaces your existing circuit breaker to protect your air conditioner against the high voltage which can occur with a ground fault.
This will NOT protect against a low voltage which can occur about 1/2 of the time with a ground-fault.
Nor will it protect against over-voltages which are not due to a ground fault.
This is what it looks like for my Square D circuit breaker box. A 20A GFCI breaker costs $65
You will have to add the 1000 ohm resistor in the circuit breaker box - between
the neutral and the ground.
note - not all CB boxes have optional GFCI breakers
What will happen when there is an open-neutral situation?
Each GFCI protected circuit with a 1000 ohm resistor in a plug somewhere on its circuit will click open, and typically the GFCI device will have a LED showing that it is open.
You can push the reset button on the GFCI device, and if the neutral is no longer open, the power will be restored to the circuit.
If the neutral is still open, the device will instantly click open again, with no harm to anything.
For readers interested in the design - why 1,000 ohms and 1/10 watt (or 1/8, 1/4, 1/2 etc)
GFCI devices open a circuit when the
difference of current between the hot and the neutral exceeds 0.005 amps
Which may be due to a 'leakage' thru a human
Normally the voltage difference between the neutral and ground in an RV is considerably less than 2 volts
A 1,000 ohm resistor would thus normally make for a difference in current = 2 volts / 1000 ohms = 0.002 amps - which the GFCI will ignore
However, during an open neutral situation,
depending on the what is turned on in the RV at the time, the neutral of a 50
amp system could range anywhere from
+ 115VAC to -115 VAC. Which would result in a piece of electronics experiencing anything from 0 VAC to 240 VAC
Voltages less than about 90 VAC will
typically result in the electronics not working,
however, voltages greater than 140 volts can burn out electronics.
When the open-neutral condition results in the neutral exceeding more than 5 volts the resistor will put more than 0.005 amps into the ground, and the GFCI device will open the circuit within 1/60 of a second.
Because a few old GFCI devices only open the hot lead rather than both the hot and the ground leads, the current could continue to flow thru the resistor after the GFCI device had clicked open. In that case, the resistor could experience up to 115 Volts for an extended period of time. Watt = V*V/R = 115 *115/1000 = 13 watts. To endure that much wattage a resistor which is too large to fit in an AC plug would be needed, so the resistor is designed to act like a fuse and burn out after a single time of activating the GFCI device. If you have this old kind of GFCI device you will need to replace the resistor after each open-ground condition. The majority of GFCI devices now open both the hot and neutral, so the 1000 ohm resistor will not burn out.
Henry Lahore March 2010
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