Battery Switch Amperage Ratings

When choosing replacement parts for customers boats it is often tempting to be the good guy and save the customer some money by installing the price leader part. The conflict usually stems from the price that the customer saw for an item in one of those discount parts books and the one you would like to install. If it's the same identical part, that can mean trouble, but, often it is not the same. One of the items that isn't often thought about is the battery switch. They're all the same? Sorry, but they are not.

The following is a brief explanation of how a switch is rated. Often the price increases as the load carrying capacity, case construction, and overall durability of the switch is accounted for. ( Much like almost everything else in the world.)  Before you buy, take a closer look.

A battery switch amperage rating means nothing without also specifying time and temperature, for the amperage rating is the current that the switch can conduct for a specified time without exceeding a specified temperature.
To compare amperage ratings between various switches there must be a common standard of time and temperature. That standard is underwriter's Laboratories (UL) Standard 1107 which defines these two important amperage ratings:

Continuous Duty Rating:
  Amperage that can be conducted for 1 hour without raising the terminal temperature more than 100 degrees C or the housing  temperature, more than 65 degrees C.

Intermittent Duty Rating:
Amperage that can be conducted for 5 minutes  without raising the terminal temperature more then 100 degrees C or the housing temperature more than 65 degrees C.

  Battery Switches which do not adhere to this standard or rate at very short time periods such as 10 seconds cannot be compared to those of manufacturers adhering to UL 1107.

One other item to look at, is the battery switch casing, how it is assembled and sealed. Since moisture corrodes the contacts of the switch, as the  switch gets older, the resistance at the contacts increases and the load carrying capacity is reduced. So, a switch of sufficient capacity as listed, when new, could be undersized as it ages.

Excerpted from: Blue Sea Systems Marine Electrical parts catalogue.
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