It's not IF, but WHEN. When will you need the protection of an EMS (Electrical Management System)? Anytime you plug into a campground or RV park, you are at the mercy of the power system in that park. Some systems leave a lot to be desired, while other systems become overloaded when everyone has two air conditioners running full-blast.

Surge suppressor vs. EMS. Most people probably know what a surge protector is - it protects against voltage surges which can occur from nearby lightning strikes or other anomalies. Some surge protectors can also test for reverse wiring and an open ground. An EMS goes a step further and monitors Voltage and Frequency, and will disconnect the RV from the power grid should they be beyond the acceptable tolerance.

Features

• Disconnects when voltage is below 104V or above 132V
• Air conditioner compressor time delay - after disconnection, can wait up to 136 seconds before reconnect
• Surge protection - 3580Joules
• Field replaceable MOV (surge protector) board
• Reverse polarity protection
• Open neutral protection
• Open ground protection
• Frequency protection - activates +/- 9Hz from 60Hz.
• 240VAC protection - if a miswired pedestal or incorrect pedestal supplies 240v to L1 or L2.

Installation is pretty much straight-forward. Essentially, you cut the inlet cable and insert the EMS between the cut ends. However, I bought a 3ft extra piece of inlet cable to give me some slack when wiring the unit. And since the attachment of the torrid coils (for ampere measurement) can be somewhat difficult due to the stiffness of the inlet cable, assembling a short piece to the output side of the EMS at the workbench is a whole lot easier than trying to do it on your back inside the RV.

The EMS systems are available in 30A or 50A versions, as well as hard-wired and plug-in. I opted for the hard-wired version as at over $300, these can easily be stolen. For the hard-wired version, you can also buy a remote monitor or on-board monitor. I recommend the remote monitor unless your EMS location is readily accessible.

Wire type. The RV industry is known for it's inconsistancy... perhaps more accurately - doing strange things to save a buck. And the entrance wiring is no exception. There are three different wiring schemes that I am aware of, and you will run across when wiring the EMS system to your RV.

In the first type, the RV manufacturer uses standard Service Entrance Cable - such as types SE/SEU/SER. These cables are somewhat smaller in diameter, but are stiffer - so they are harder to bend and install. They also may not work well with the 1 1/4" cable clamps that ship with the EMS box, so you may need to replace the clamps with smaller ones.

Another type of installation RV manufacturers use is to simply use a length of the same wiring (STW or similar) used for the extension cable to connect between the power inlet and breaker panel. The diameter of this wiring is usually larger and the 1 1/4" clamps should adequately hold this cable, so you should not have to replace the clamps.

And a third practice RV manufacturers use is to simply directly connect the power cable to the breaker panel without an inlet connector. Of course this is a less expensive approach, but the 1 1/4" clamps should work here too.

The result is when installing an EMS system, various wire sizes may be encountered.

Since my RV manufacturer used the service entrance type wire, the cable clamps were a bit too large, and did not adequately secure the cable. So I replaced them with Cord Grips as shown here:

You can probably buy the reducer washer and locknut at your local electrical supply house, but the Heyco cordgrips are harder to find. Also, the Chinese knockoff versions (UXCell etc) will not likely work as they have a shorter threaded section. Of course, you can use the more traditional cable clamps in the 1" size as well. In summary, the cable type your RV manufacturer uses for the inlet power will dictate if you should change out the clamps.

The hardware including standoffs should be able to be found at any good hardware store. The front panel was ordered from Front Panel Express using the template found on my Template Page. Download template EMS-1.

You will find that one issue with flush-mounting is there is not a lot of room behind the panel for the modular connector. Depending on the thickness of the mounting board, you may have to cut a notch or slot on the backside for clearing the connector. This can easily be done with a oscillating multitool cutter.

The remote will continuously display the current error condition, voltage, and current. For the 50Amp version, voltage and current for both legs (L1 and L2) will be displayed.

• E 0: Normal Operation
• E 1: Reverse Polarity
• E 2: Open Ground
• E 3: Line 1 High Voltage (above 132V)
• E 4: Line 1 Low Voltage (below 104V)
• E 5: Line 2 High Voltage (above 132V)
• E 6: Line 2 Low Voltage (below 104V)
• E 7: Line Frequency High (above 69Hz)
• E 8: Line Frequency Low (above 51Hz)
• E 9: Data Link Down
• E10: Replace Surge Suppressor

  "PE" Errors

  Sooner or later, you will get a PE error. This happened to us within 2 weeks of installing the EMS. We had a thunderstorm roll through and we lost power to the RV. When power was returned, we received a PE4 error right after the E0 status (Normal Operation). PE as it turns out, means "PREVIOUS ERROR", and 4 is the actual error - in this case, Low Voltage. The number after PE means the error you experienced. So PE4 means a previous, but cleared Low Voltage error.

  So the EMS did it's job. It sensed a low voltage condition during or just prior to the loss of power, and shut the RV down.

  PE Errors are automatically cleared when power cycling the RV from the power pedestal.

   

References:

NEMA Bulletin 71 Standard Electrical Knockout Sizes.