Mobile Installation Power Ground

 

On the Technician class test there is a question which says, “What is the purpose of a fuse in an electrical circuit?” and the answer is, “To interrupt power in case of overload.”  In the Gordon West Technician Class Q And A book

Curtis, KF6VFP in his pickup with a magmount for mobile operation and Wade, KN6WX

it says, in the added notes, “Place the fuse as close to the battery as possible.” (Page 162).  That is good advice because if for any reason the positive wire’s insulation is damaged and the wire touches the chassis or engine (a hot manifold is a frequent cause of this problem) then it will blow the fuse if it is close to the battery because the fuse will be between the short and the battery.   If the fuse is close to the radio it is more likely to be between the radio and the short and thus not open which could start a fire in the vehicle.  On that same page it shows a schematic of an installation and it shows a fuse in the negative line.  Many hams think that is unnecessary because you will not cause a short if the negative wire touches the metal of the vehicle.  So why place a fuse in this line?

The diagram shows the negative wire going straight back to the negative terminal of the battery.  What I am about to say I have personally seen happen more then once so it is not hypothetical.  If the battery cable develops resistance between the cable and the body of the vehicle by rust or corrosion or the wire itself corrodes to the point that it is not a good conductor this type of installation can cause problems.

When the engine is being started a lot of current is being drawn from the battery and the wiring to the mobile radio is not designed to handle any where that much current.  Simple Ohm’s law will tell you that the maximum current will flow through the path of least resistance and if that path happens to be through the negative wire of the radio to the negative terminal of the battery then that is the where the most current will flow.  Frequently the unit is not grounded well at the mounting bracket but the shield side of the coax makes a good ground by the antenna mount.  In that case the current for the starter will attempt to flow through the coax shield to the coax connector on the radio then on to the negative wire to the battery. If that wire is not fused the coax shield will smoke. If the radio is grounded at the mount the negative wire to the battery is not big enough to handle the load and it will smoke.  Either way there is a fire danger.  The resistance between the battery and the vehicle chassis does not have to be high enough that the vehicle will not start to cause this phenomenon.    Thus with this type of installation a fuse is used near the battery.

Personally I do not recommend this type of hookup.  If you do this and the fuse blows you may not know it is blown because the radio finds sufficient contact

The negative wire going back to the radio is X (crossed out) because that wire should go directly to the vehicle’s chassis like the negative wire does for the vehicle’s negative ground.

between the mounting bracket and/or the antenna ground to continue to operate.  The antenna ground and the mounting bracket are not designed to be the negative source for the DC power of the radio and it will cause more problems then you can imagine.  Believe me as a two way radio technician for many years I have seen these problems and they can drive you crazy.  I suggest that you run the ground wire to the chassis of the vehicle.  Use an eye connector with one outer locking lock washer between the head of the screw and the lug and another between the lug and the chassis.  Scrape the paint off the place where the lug will come in contact with the metal.  Run the screw down tight but do not strip it out.

The radio was not designed to get its ground through the mounting bracket or through shield side of the coax which is the only way it can find the negative terminal of the battery if the fuse on the negative wire is blown.  I can not tell you how many problems I have found that were caused by a poor ground connection.  By using proper installation processes your mobile radio should run failure free for the life to the vehicle or installation.

 

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One Response to Mobile Installation Power Ground

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