Lost or Open Neutral Presentation

[danemayer]

Sometimes we hear about the serious consequences of losing the neutral line on our 50amp power service. Voltage can go way above 120V on one leg of the power system, and way below 120V on the other. I've posted a presentation (with permission from the author) that steps through an example explaining exactly what happens and why.

It's the Lost Neutral Presentation on this page.

 

[porthole]

Yeah - that ought to generate some conversations

 

[evolvingpowercat]

Yes it is possible that shore power pedestal L1 and L2 power sources will be opposite phase 120 volts and that you can read 240 volts with a meter between the L1 and L2 sockets on the shore cable receptacle. This is allowed by electrical codes but not required. If neutral is open if shore power is wired this way there can be high voltage on one L side and low voltage on the other L side of your circuits.

TRC Surge Guard 34560 or several other similar protection devices will not allow power connection if neutral is open or if L1 or L2 voltage is outside the 100 - 130 volt range. The hard wired type like the 34560 will not only protect against bad power at the pedestal but in the unlikely event neutral came open in your shore cable or TT cabling between shore cable and the surge guard it would protect against that, too.

 

[dave10a]

Beware that the differential of the voltage between l1 and l2 when neutral is lost or high resistance is determined by the load or current draw. This means the voltage will appear normal between l1 and l2 when there is no load and and increase towards 220vac on l1 or l2 as the load is increased. A 200-500 watt load would most likely be adequate to test the voltages.