Re: Headlamp circuit breaker mount

Posted by DavidPackard On 2016/5/11 17:41:09

To use household electrical terms the copper binding post is the 'LINE', while the silver post is the 'LOAD'. In automotive terms the 'HOT' wire from the battery (for our 6 volt positive ground cars this would be the negative lead) attaches to the copper post, and the silver is attached to the headlight switch. The stationary post is identified by a copper plating, or a shorter length. The 'Packard' OEM circuit breakers are also marked. The short post is marked 'BAT'. The long post is marked 'AUX'
Internally a circuit breaker is somewhat like a set of points. When all is well the points are in contact, thus providing a circuit. One side of the point set is fixed in the insulated base of the circuit breaker, while the other is mounted on a leaf. It is the leaf that responds to the heat that varies with current. When the circuit breaker 'pops' the leaf defects and opens the circuit. Automotive headlight circuits utilize what is known as a 'Type I', or automatic reset breaker. The leaf motion must be abrupt (snap action) to avoid excessive arcing.
It is a safer design that fixes the 'HOT' lead, and allows the load side to defect. The concern is the failure mode where the leaf defects in such a way that contacts the external case. If the leaf defects into the case, and the circuit breaker was installed correctly there is no short circuit . . . the leaf is not 'HOT'. Just as long as the points re-align as the unit cools this would be known as a 'fault tolerant' design.
The circuit breaker doesn't know about positive versus negative ground circuits, and will work 'OK' with either post attached to the headlight switch, but it is safer to conform to the design intent.
There are European designs that use fuses versus circuit breakers in the headlight circuits. This got me to thinking about what type of failure mode are we trying to protect from that the very first response is to re-energize the circuit . . . and try again . . . and again . . . and again.
The battery is connected to the copper, or, if the posts are not 'color' coded, the shorter post. The brass plate that is part of the light switch is attached to the silver or longer post.
dp