The EMS has a big relay (contactor) that actually does the switching of the high current AC power. It is common for the relay contact points to have arcing across them if an electrical load is present when the relay switches. This arcing can cause a failure of the relay to pass power.
To lessen the incidence of EMS relay contacts arcing, I follow an arrival electrical hookup/departure electrical disconnect routine designed to minimize this. It actually starts with the previous departure where I turn off the inside mains breakers BEFORE turning off/disconnecting shore power. This causes the load associated arcing to be in the inside mains breakers which are usually available at hardware stores (not like EMS systems or EMS relays). When connecting up at a new RVing site, I do the reverse - connect up the shore power cord, turn on the park pedestal breaker, and LASTLY turn on the inside mains breakers. This causes the EMS relay to switch with NO LOAD on it, and preserves the EMS relay contacts life. Of course, if you have an electrical problems during your stay, and the EMS relay switches the power off, some contact arcing is going to happen.
I had a problem a couple of years ago with my EMS giving false readouts (255 volts on a 30 amp 120 volt circuit) and shutting the power down during a high heat wave. Evidently the computer board had fried. I called Progressive, related to them that I was a retired electronics tech of 40 years experience. They sent a new circuit board to my site and let me install it. I found 2 capacitors at the bottom of the bad board fried. While I had the phenolic cover off of the EMS box, I got out my trusty Cramolyn DeOxideIT contact restorer spray and a flat swiss file to dress and enhance the relay contacts. I also thought that the high heat and sealed box may have overheated the capacitors that failed, so I drilled a row of about 5 1/8 inch holes under the length of the circuit board and directly above the circuit board in the cover to allow some cooling convection air flow across the PC board while maintaining MOST of the watertight integrity of the phenolic enclosure box. I also spaced the entire box off of the floor with machine nuts under the mounting screws, to allow this air flow.
So far no more problems with the EMS box, and it has cut power under bad electrical supply problems several times during the past couple of years. This summer it was cutting off the power when 240 volts was being measured on the neutral, 120 volts on the hot (due to a park transformer neutral-ground bonding problem), while the other RVers in the park section kept using their power unknowing of this dangerous condition.
