RVs primarily have AC problems with open-neutral, which is also known as a ground fault.
As with houses, RVs rarely have a problem with lightning surges or other high voltages surges - which are protected by outlet strips.
The open-neutral conditions can occur for most RVs when plugging into 'shore power' if the neutral is not connected to ground.
this open-neutral connection can occur at a variety of locations for all RVs: at the power stand, in the park network, in the cable from the RV, etc.
The open-neutral problem can also occur on RVs which have transfer switches to go between shore power and generator power (such as Onan).
The transfer switch can have an open-neutral for a tiny fraction of a second if it is not designed to switch the neutral BEFORE switching the power.
Even plugging into shore power with the circuit breaker on can cause a momentary open neutral. It's almost impossible to connect, or disconnect, that plug in such a manner that all 4 terminals make contact simultaneously.
The transfer switch using relays can also result in a long time open-neutral if it fails to connect the neutral due to dirty contacts, weak springs, etc.
This was the case for a motorhome I was traveling with in Sept 2009 - see below
The open neutral condition will burn out electronics and perhaps even burn the RV only if the RV is connected via a 50A outlet.
30A outlets has a a maximum of 120 volts, and during an open-neutral situation only 0 volts.
50A outlet has a maximum of 240 volts see graph below
during an open neutral situation virtually all 240 volts will be applied to one of the 120 volt circuits - burning out many types of electronics
Another common AC problem is low voltage. When the 120V power falls below 108V, any large motors, such as in your air conditioner, can heat up and draw more and more current until it eventually damages the motor. the ground fault protection does not deal with that problem, but most surge protection systems do.
High-cost protection if NOT HAVE on-board generator $200 - $400
Surge protection, which can be used outside or wired into the RV, can protect against the common open neutral situation and many other problems
High-cost protection if you HAVE on-board generator - and transfer switch $400- $700
Use an internal surge protector - to protect against open-neutral conditions as described below
1) Stand-alone surge protector such as Surge Guard 40240, Progressive EMS HW,
2) Surge protector and transfer switch combination
- such as Surge Guard 40250 or Intellect 00-01034-200 - not yet sold as of Sept 09
************ the following are my notes on diagnosing a open-neutral situation ****************
Sept 2009 When plugged motorhome into 30A outlet (120 volts max, not 240 volts)
The series of connections was as follows:
box < 30A to 50A adapter < Surge protector < 50 A cable < transfer switch < circuit breaker box
I measured 6 voltages (at 5 locations) with the above setup.
1 inside circuit breaker box in motorhome
between neutral and ground (green = bare copper = metal box)
voltmeter and neon tester both showed 120 volts between neutral and ground
should have been 0 volts
2 at 30A power outlet near the motorhome: 0 Volts between screw and round pin
3 at 50A connector coming out of the standalone Surge Protector. Then I opened up the transfer switch, and this may have resulted in re-seating the neural contacts
4 at the input to the transfer switch. 0 volts between neutral wire and ground
5 at the output from the transfer switch 0 volts between neural wire and ground
6 inside the circuit breaker box (same measurement as #1 above)
this time it was 0 volts as it should have been originally
Conclusion: The neutral is not always connected to ground as it must in order to have two separate 120 volt circuits.
If the neutral is open (not connected to ground) and a motorhome is connected to the 50A outlet there can be an over-voltage situation. Depending on the loads on the two different 120 volt circuits at the time the voltage on one of the lines could have been as high as 240 volts for an extended period of time.
A motorhome/RV is normally connected to a 50A shore outlet,
On Sept 16, 2009 the motorhome was plugged into a 50 amp shore outlet and it appears that some portion of the coach was subjected to high voltage due to lack of a neutral. The Xantrix inverter/charger and NorCold freezer appear to have not survived the event. A quick survey found no other electrical loads have died.
I expect that the Xantrix has an internal (not user serviceable) fuse which can be replaced. The NorCold is able to still operate on its second source of power 12V DC, but no longer operates on 120 VAC. NorCold personnel indicated that the 120V circuitry inside the freezer is fairly simple, so it might be fixed by replacing the rectifiers and capacitors.
The transfer switch design, which appears to be used by have of the products available, has a design flaw. The normal/unenergized position of the transfer switch connects the neural to the shore ground. It appears that the the spring in the relay is the only thing which assures that the contact will be made. If there is any dirt or corrosion then the contact would not be made. This is a common failure mode for relays. This can be minimized by 1) having a relay with multiple contacts, 2) having a relay with wiping contacts (so that the contacts are self-cleaning). The problem could be eliminated by using a surge protector after/downstream of the transfer switch. Note: one function of the surge protector is to detect an ground fault (open neutral) and not pass the resulting overvoltage condition through to the AC loads in the motorhome/rv.
I hope, but have no way of knowing, that the relay sequences the contacts such that the neutral contact is made before the red and black contacts. If not, there could be a high voltage applied to the motorhome every time that it is plugged into shore power (for perhaps 1/100th of a second)
By Henry Lahore, retired Electrical Engineer.
For several years I did failure modes and effects analysis for Boeing,
and helped make the computerized failure management system for the 777 aircraft.
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