A switch can be constructed with any mechanism bringing two
conductors into contact with each other in a controlled manner. This can be
as simple as allowing two copper wires to touch each other by the motion of
a lever, or by directly pushing two metal strips into contact. However, a
good switch design must be rugged and reliable, and avoid presenting the
operator with the possibility of electric shock. Therefore, industrial
switch designs are rarely this crude.
The conductive parts in a switch used to make and break the electrical
connection are called contacts. Contacts are typically made of silver
or silver-cadmium alloy, whose conductive properties are not significantly
compromised by surface corrosion or oxidation. Gold contacts exhibit the
best corrosion resistance, but are limited in current-carrying capacity and
may "cold weld" if brought together with high mechanical force. Whatever the
choice of metal, the switch contacts are guided by a mechanism ensuring
square and even contact, for maximum reliability and minimum resistance.
Contacts such as these can be constructed to handle extremely large
amounts of electric current, up to thousands of amps in some cases. The
limiting factors for switch contact ampacity are as follows:
Heat generated by current through metal contacts (while closed).
Sparking caused when contacts are opened or closed.
The voltage across open switch contacts (potential of current jumping
across the gap).
One major disadvantage of standard switch contacts is the exposure of the
contacts to the surrounding atmosphere. In a nice, clean, control-room
environment, this is generally not a problem. However, most industrial
environments are not this benign. The presence of corrosive chemicals in the
air can cause contacts to deteriorate and fail prematurely. Even more
troublesome is the possibility of regular contact sparking causing flammable
or explosive chemicals to ignite.
When such environmental concerns exist, other types of contacts can be
considered for small switches. These other types of contacts are sealed from
contact with the outside air, and therefore do not suffer the same exposure
problems that standard contacts do.
A common type of sealed-contact switch is the mercury switch. Mercury is
a metallic element, liquid at room temperature. Being a metal, it possesses
excellent conductive properties. Being a liquid, it can be brought into
contact with metal probes (to close a circuit) inside of a sealed chamber
simply by tilting the chamber so that the probes are on the bottom. Many
industrial switches use small glass tubes containing mercury which are
tilted one way to close the contact, and tilted another way to open. Aside
from the problems of tube breakage and spilling mercury (which is a toxic
material), and susceptibility to vibration, these devices are an excellent
alternative to open-air switch contacts wherever environmental exposure
problems are a concern.
Here, a mercury switch (often called a tilt switch) is shown in
the open position, where the mercury is out of contact with the two metal
contacts at the other end of the glass bulb:
Here, the same switch is shown in the closed position.
Gravity now holds the liquid mercury in contact with the two metal contacts,
providing electrical continuity from one to the other:
Mercury switch contacts are impractical to build in large
sizes, and so you will typically find such contacts rated at no more than a
few amps, and no more than 120 volts. There are exceptions, of course, but
these are common limits.
Another sealed-contact type of switch is the magnetic reed switch. Like
the mercury switch, a reed switch's contacts are located inside a sealed
tube. Unlike the mercury switch which uses liquid metal as the contact
medium, the reed switch is simply a pair of very thin, magnetic, metal
strips (hence the name "reed") which are brought into contact with each
other by applying a strong magnetic field outside the sealed tube. The
source of the magnetic field in this type of switch is usually a permanent
magnet, moved closer to or further away from the tube by the actuating
mechanism. Due to the small size of the reeds, this type of contact is
typically rated at lower currents and voltages than the average mercury
switch. However, reed switches typically handle vibration better than
mercury contacts, because there is no liquid inside the tube to splash
around.
It is common to find general-purpose switch contact voltage and current
ratings to be greater on any given switch or relay if the electric power
being switched is AC instead of DC. The reason for this is the
self-extinguishing tendency of an alternating-current arc across an air gap.
Because 60 Hz power line current actually stops and reverses direction 120
times per second, there are many opportunities for the ionized air of an arc
to lose enough temperature to stop conducting current, to the point where
the arc will not re-start on the next voltage peak. DC, on the other hand,
is a continuous, uninterrupted flow of electrons which tends to maintain an
arc across an air gap much better. Therefore, switch contacts of any kind
incur more wear when switching a given value of direct current than for the
same value of alternating current. The problem of switching DC is
exaggerated when the load has a significant amount of inductance, as there
will be very high voltages generated across the switch's contacts when the
circuit is opened (the inductor doing its best to maintain circuit current
at the same magnitude as when the switch was closed).
With both AC and DC, contact arcing can be minimized with the addition of
a "snubber" circuit (a capacitor and resistor wired in series) in parallel
with the contact, like this:
A sudden rise in voltage across the switch contact caused
by the contact opening will be tempered by the capacitor's charging action
(the capacitor opposing the increase in voltage by drawing current). The
resistor limits the amount of current that the capacitor will discharge
through the contact when it closes again. If the resistor were not there,
the capacitor might actually make the arcing during contact closure worse
than the arcing during contact opening without a capacitor! While this
addition to the circuit helps mitigate contact arcing, it is not without
disadvantage: a prime consideration is the possibility of a failed (shorted)
capacitor/resistor combination providing a path for electrons to flow
through the circuit at all times, even when the contact is open and current
is not desired. The risk of this failure, and the severity of the resulting
consequences must be considered against the increased contact wear (and
inevitable contact failure) without the snubber circuit.
The use of snubbers in DC switch circuits is nothing new: automobile
manufacturers have been doing this for years on engine ignition systems,
minimizing the arcing across the switch contact "points" in the distributor
with a small capacitor called a condenser. As any mechanic can tell
you, the service life of the distributor's "points" is directly related to
how well the condenser is functioning.
With all this discussion concerning the reduction of switch contact
arcing, one might be led to think that less current is always better for a
mechanical switch. This, however, is not necessarily so. It has been found
that a small amount of periodic arcing can actually be good for the switch
contacts, because it keeps the contact faces free from small amounts of dirt
and corrosion. If a mechanical switch contact is operated with too little
current, the contacts will tend to accumulate excessive resistance and may
fail prematurely! This minimum amount of electric current necessary to keep
a mechanical switch contact in good health is called the wetting current.
Normally, a switch's wetting current rating is far below its maximum
current rating, and well below its normal operating current load in a
properly designed system. However, there are applications where a mechanical
switch contact may be required to routinely handle currents below normal
wetting current limits (for instance, if a mechanical selector switch needs
to open or close a digital logic or analog electronic circuit where the
current value is extremely small). In these applications, is it highly
recommended that gold-plated switch contacts be specified. Gold is a "noble"
metal and does not corrode as other metals will. Such contacts have
extremely low wetting current requirements as a result. Normal silver or
copper alloy contacts will not provide reliable operation if used in such
low-current service!
REVIEW:
The parts of a switch responsible for making and breaking electrical
continuity are called the "contacts." Usually made of corrosion-resistant
metal alloy, contacts are made to touch each other by a mechanism which
helps maintain proper alignment and spacing.
Mercury switches use a slug of liquid mercury metal as a moving
contact. Sealed in a glass tube, the mercury contact's spark is sealed
from the outside environment, making this type of switch ideally suited
for atmospheres potentially harboring explosive vapors.
Reed switches are another type of sealed-contact device, contact being
made by two thin metal "reeds" inside a glass tube, brought together by
the influence of an external magnetic field.
Switch contacts suffer greater duress switching DC than AC. This is
primarily due to the self-extinguishing nature of an AC arc.
A resistor-capacitor network called a "snubber" can be connected in
parallel with a switch contact to reduce contact arcing.
Wetting current is the minimum amount of electric current
necessary for a switch contact to carry in order for it to be
self-cleaning. Normally this value is far below the switch's maximum
current rating.
|