Expert: Ernest (Ernie) Kenward - 11/5/2008

Question
Question 18: Faulty switches on older cars can often be caused by:

A: a lack of lubricant
B*: no relay used (Why is this answer right?)
C: poor quality switches
D: underloaded circuits


Answer
My usual pat answer to the failures of any older equipment comes down to the ravages of Time and Temperature over the Long Term!

Mechanisms and electrical insulation do not last forever.  Mechanisms fail due to fatigue of metals and plastics, often exacerbated by lubrication not being refreshed at in reasonable intervals.

Electrical contacts fail due to pitting caused by arcing during load breaks and occasional short circuits.  

Insulation fails owing to drying out; temperature rise accelerates this process, the higher the T, the faster the breakdown.  Most insulation is based on organic materials, thermoplastics being carbon-based, for example.  While the engineered thermoplastic can have excellent insulation and dielectric characteristics, it loses these as it deteriorates back to its base materials, and carbon itself is quite a good conductor.  Any insulation in use WILL fail eventually - this is because the localised breakdown of the material will begin due to (i) higher temperature in that section of the conductor, and/or (ii) leakage current caused by a voltage transient, and (iii) enough time to dry out due to ambient temperature and the surrounding environment.   Leakage current in the incipient stage can be very small, but the material in its locale has returned to its base carbon form.  It will heat due to its own resistance to the flow of current, and cause further carbonisation around it.  The leakage current will increase, further carbonisation will occur, and this will ultimately cascade into a full-scale fault.

This is more information than you requested, but it describes the mechanics behind eventual electrical failure.

As to why switches driving relays (power contactors, really) tend NOT to fail as soon (or within the timeframe of our lives, perhaps!) is because it is now the relay, and not the switch, which is carrying the load current; the switch need carry only enough current to drive the relay's coil.  If the switch happens to be one which was originally intended to handle the load current itself, then using it to drive a contactor coil instead will be very light duty, and this therefore will greatly prolong the life of the switch contacts.  The same, of course, would not be true for the contactor!  However, the contactor may either be heavier duty in its design or, instead, just as fragile as a switch but more easily replaceable, by being a plug-in component, for example.

Incidentally, the only problem with a switch originally intended to drive a power load being used to drive the much lesser load of a relay coil is that - sometimes - too LITTLE current might not be a good thing.  Switch contacts often rely on the small amount of arcing as they break a load to burn off the oxides that tend to form on switch contacts; since this happens each time a switch opens, the oxide has little chance to build up.  However, should the contact current be insufficient to produce that small "cleansing" arc, as it might be when driving a small coil (or a programmable controller PLC input) is that the oxide may build until the switch contacts become intermittent and unreliable.  

This is a common problem that can happen down the road from relay or PLC upgrades if one does not anticipate the problem and act to solve it before it occurs.  The fix is to specify the correct type of switch for the task.  Most industrial- and automotive-duty contacts are nickel-plated to resist corrosion and also be resistant to arcing, as they directly switch larger resistive and inductive load currents.  The correct contacts for low-current (mA range) switching are gold-plated - very thin plating.  Gold is very high on the list of electrically-conductive metals, and also very resistant to corrosion.  Gold contact surfaces will stay oxide-free, and continue to pass mA of current with no need (and no desire!) for a cleansing arc.

And that is why industrial switch catalogs provide choices in available contact materials.

Thanks for a good question!

Regards ... EGK