The ZNRs I installed on my hydraulic pump were Panasonic ERZ-V20D220 with a varistor voltage of 20V and a Maximum Allowable Voltage of 18V. These will take a 3,000A surge which is probably overkill but size, weight, and cost weren't really a consideration for me; Longevity was. I chose not to use the part number with the next lower (14V) Maximum Allowable Voltage because the Panasonic ERZ-V20D180 could possibly have conducted at normal battery voltage when taking the 10% tolerance into account.

In reading further about protecting switch contacts, some companies recommend using a resistor and capacitor in addition to the MOV. You can read about that at Littelfuse Varistor Design Examples "CONTACT ARCING DUE TO INDUCTIVE LOAD".

Each time the current in the inductive load

is interrupted by the mechanical contacts,

the voltage across the contacts builds up as -

L di/dt. When the contacts arc, the voltage

across the arc decreases and the current in

the coil can increase somewhat.The extin-

guishing of the arc causes an additional

voltage transient which can again cause the

contacts to arc. It is not unusual for the

restriking to occur several times with the

total energy in the arc several times that

which was originally stored in the inductive

load. It is this repetitive arcing that is so

destructive to the contacts.

Littelfuse recommends installing both an MOV and a series resistor and capacitor in series across the contacts. I really don't have the proper test equipment to determine the proper values of the R and C, though. It would be a worthy project for someone to do some testing to determine the best protection scheme for the hydraulic pump relays, but I don't really have the time or the equipment at the moment.

I'm also wondering what the inrush current is on the hydraulic pump motor. Inrush can weld relay contacts as well as arcing. I think I can use my Fluke in the "peak hold" mode to measure the current across the battery shunt as the hydraulic pump kicks on.

I also installed two of those same ZNRs across my starter coil contactor - one from the input terminal to the disc and another from the output terminal to the disc. Those protect the contactor from arcing, but I need to add two more from the input and output terminals to ground to protect the starter switch on the panel.

Safety tip: Be aware that when these MOVs fail due to excessive load, they can fail shorted. In that case they can either catch fire or explode. In consumer electronic equipment, I have seen them covered with a fiberglass "fire sleeve" in order to meet UL requirements. It's something to think about...