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Spikes & Showering Arcs

Is Your Carwash Electronic Equipment Protected?

 

IDX Application Note 101
 

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.