Battery Switch Ratings Explained

March, 07 2002

There is only one objective standard by which marine battery switches are rated and that is the Underwriter's Laboratories (UL) UL 1107 standard. However, as of this writing this standard is not a formal UL a standard at all. To quote UL, "UL 1107 will soon be an Outline of Investigation which is a fancy way of saying 'an internal standard used by us that is not subject to industry updates and file reviews but will be available for distribution'".

The key part of this "standard" is the amperage ratings test, which establishes two ratings:

Intermittent RatingIntermittent is a 5-minute rating and is based on temperature rise of various sections of the switch as the rated current is applied over a 5-minute period.

Continuous RatingThe Continuous rating is the same, but the time period is 1 hour. This is considered a long enough period to be called "Continuous" because by this time the switch has reached temperature equilibrium, that is, the rate at which the amperage flow is generating heat is equivalent to the rate at which the switch is dissipating that heat.

As these standards demonstrate, there are three variables involved in rating battery switches: time, current and temperature. For any given switch the relationship of the variables is TEMPERATURE = TIME x CURRENT. More current equals more heat. A longer time period equals more heat. More time and more current generates lots more heat!

A problem with these ratings is that marine battery switches are commonly used in the engine starting circuit and neither the Continuous nor the Intermittent Ratings represent the conditions a switch endures in the engine starting circuit, typically 10 seconds or less, but at very high currents. The ABYC Standards have propagated the problem by requiring that a switch in the engine cranking circuit be rated such that its Intermittent Rating is at least equal to the highest engine starting current. Peak starting currents can easily spike to 1500 amperes, yet there are almost no marine battery switches rated at this Intermittent Current.

Some foreign manufacturers have avoided testing to the UL standard at all and some of these have avoided specifying the time periods at which their ratings claims are made or have stated ridiculously short time periods and omitted mentioning how their switches performed in longer 5 minute and 1 hour situations. This has only fueled the confusion by making it appear that these switches are of much higher capacity that their UL tested counterparts.

In the future a separate Technical Brief explains the Blue Sea Systems' Engine Starting Standard that establishes a realistic method of determining a switch's suitability for starting circuits.

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