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Overview
There are two primary sources for electrical interference
of electronic equipment: a) lightning strikes, and b) switching power
to inductive loads such as motors, contactors, solenoids, and
transformers. The voltage spikes and high frequency noise
generated by these sources can disrupt the operation of, or even
damage electronic controls such as timers, acceptors, displays
etc. Line voltage spikes on your power mains (120VAC or 240VAC) are
generally transformed nicely right through your 24VAC transformers and
delivered to your electronic equipment, and electrical disturbances
placed on your 24VAC power at one location, will show up directly on
other equipment powered by the same transformer.... unless you take
specific action to clamp and absorb this disruptive and destructive
energy.
Voltage Spikes
 Voltage
spikes are very brief disturbances that are very dangerous for your
electronic equipment because the voltage values may reach thousands of
volts... even on your 24VAC transformers. They are caused, not only by
the switching of high voltage lines, connection of power factor
correction capacitors, lightening, and the switching off of inductive
load equipment such as motors, solenoids, and contactors that also
feed your electronic control equipment. Spikes are not
detectable by means of an ordinary voltmeter given their brief
duration; however they are one of the main causes of faults and
malfunctions.
Switch/Relay Contact
Spikes
When an electrical contact that was providing current to an inductive
load is opened, the current instantaneously stops, but the collapsing
magnetic fields in the inductive load create a back-EMF voltage spike.
Depending on the characteristics of the equipment and local wiring,
either a single large voltage spike will be generated, or the air
between the separating contacts may actually break down from the
rising voltage creating a showering arc, as shown later in this
document. A voltage spike of a thousand volts lasting for only about a
millisecond or so would not be an uncommon thing to be generated when
switching off a solenoid. Although most power supplies on electronic
equipment can absorb this spike without damage, it also can generate a
spike of current in the power supply circuit of the equipment that may
upset other portions of the circuit... such as sensors reading
temperature, examining coins, reading credit cards resulting in
strange behavior... or even worse, it may upset the microcontroller or
its memory circuits, again resulting in strange behavior. These spikes
are best clamped by Transorbs or MOVs such as are built into
IDX Snubber Terminal Blocks.
Lightening Strike
Spikes
When lightening strikes ground, there is an instant surge of upwards
of 100,000 Amps of current into the ground at that point. Our
electrical distribution system uses the massive and deep earth to
provide the return electrical path in our power system... thus the
name ground potential. If the lightening strikes near a power
substation, the ground potential right there will suddenly spike up...
on top of which is the transformer voltage sent over the power lines
to you.... where your ground potential is not the same as the spiking
ground potential at the substation. Thus, relative your nice quiet
ground potential, the normal AC power suddenly appears to have this
big spike on it. Normally, spike voltages in your breaker box are
limited to about 6,000V by gap distances between hot and ground, thus
limiting what you see to "only" a 6,000V spike. Your 5:1 step down
transformer from 120VAC to 24VAC will pass this to your equipment as
"only" a 1,200V spike.... yikes! These spikes are best clamped on the
secondary side of the transformer by Transorbs or MOVs such as are
built into IDX Snubber Terminal Blocks.
Showering Arc Spikes
 As
described above, when an electrical contact that was providing current
to an inductive load is opened, the current instantaneously stops, but
the collapsing magnetic fields in the inductive load create a back-EMF
voltage spike. In some cases there will not be a clean single spike
generated, but instead the voltage will create a very high frequency
arc, known as a showering arc, between the opening contacts until all
of the back-EMF energy is dissipated. The showering arc is caused by a
relaxation oscillator effect due to the inductance of the load, stray
circuit capacitance, and the non-linear characteristics of the air gap
during electrical breakdown for the arc. As shown in the figure above,
the showering arc can be many hundreds of volts and contain
frequencies in the hundreds of kilohertz to low megahertz. This kind
of electrical interference is particularly nasty for sensors
attempting to read the metal alloy of coins, read credit card magnetic
stripes, sensitive infrared receivers for configuring products, and
possibly even input signals to various control equipment. Again, these
spikes within the showering arc best clamped and absorbed Transorbs or
MOVs such as are built into IDX Snubber
Terminal Blocks.
Application Of MOVs Or Transorbs
These energy absorbing components can be applied in several ways.
Because they are generated by a combination of opening contacts and an
inductive load, they can be placed across the terminals of the
inductive load or across the contacts of the switch. In a 24VAC
system, a 14mm disk MOV with a clamping voltage of about 39VDC will
perform adequately. Alternatively, if there is some particular
equipment that appears to be susceptible to an unknown noise source,
the MOV or Transorb may be applied right to the power supply terminals
of the equipment having problems.
If you have unprotected switch contacts on a
contactor or rotary selector switch, you can often diagnose the what
the source of your interference is by repetitively making and breaking
the contacts that control various equipment and observing the
susceptible equipment for a cause and effect relationship. |
