The voltage and current in a complete electrical circuit obey Kirchhof’s voltage and current laws.  These laws simply stated are:  the rises and drops in voltage around any closed circuit (a circuit loop) must sum to zero; and the total current flow into any one junction (connection point) must also sum to zero.  If we wish to interrupt the current in a circuit, we must do so in accordance with these laws.

Although it sounds simple, interrupt the circuit, break the conduction path, or open the switch – it is not.  Forcing a conducting circuit to a steady-state condition of zero current is anything but simple.  Many times, the actual detailed physics of the process of current interruption is obscured by the seeming triviality of the switching action – such as simply flicking off a flashlight.  But consider what actually happens when a flashlight is turned off.

A steady-state direct current (DC) is flowing from the batteries to the bulb as the switch contacts begin to move.  At the last microscopic points of electrical contact, the current density becomes high enough that portions of the metallic surfaces actually melt due to resistive heating; and a liquid metal vapor plasma state continues the electrical conducting path as the contacts physically part.  As the contacts pull further apart to distances of several microns (one micron = 10^-6 meters), electrons from the contact into which the current is flowing, the cathode contact, are emitted into the intercontact space region due to thermal emission (they boil off) and field emission (they are ripped from the cathode metal by electrostatic attraction forces).

A portion of these electrons emitted from the cathode collide with air molecules within the contact gap and ionize the molecules. This frees still more electrons, which in turn ionize still more air molecules.  This self-perpetuating action is an electrical breakdown phenomenon commonly referred to as an arc.  It is the arc which enables the switch to open the circuit.  The arc forms just as the contacts part, and continues to conduct the circuit current as the contacts move further and further apart.

The voltage drop across the arc – which is proportional to the arc length and inversely proportional to the arc cross-sectional size – is in series with the voltages in the circuit loop which contains the switch.  The arc voltage grows as the arc is lengthened by the physical movement of the contacts, and the arc cross-section is diminished as the arc is cooled by contact with un-ionized air molecules.

The arc voltage in low voltage DC circuits grows at such a rate that it soon exceeds, or at least matches, the source voltage in the circuit (in a flashlight the initial arc voltage exceeds the battery voltage).  When this occurs, the circuit current is driven to zero in short order.  All circuits contain a small but finite inductance, so the current cannot be driven to zero instantaneously.  When the current does reach zero, no further arc ionization takes place, and the arc is cooled even more rapidly, since it has no energy input.  If it is cooled momentarily to such a state that it is no longer a conducting medium, then the interruption process is complete and the circuit has been opened.  It is important to remember that it is the arc that forces the current to zero.  The opening of the switch forms the arc, but it is the arc which enables the circuit to be interrupted.

A switch or circuit interruption device which is intended to open alternating current (AC) circuits has a somewhat easier chore than its DC counterpart.  In AC circuits, there is no need to force a current-zero condition.  Since the current alternates about zero already, there is a natural current-zero twice in each AC cycle.  Any arc which forms in an AC switching device does not have to be stretched and cooled to the extent that the arc voltage exceeds the magnitude of the circuit source voltage.   However, this can be done if one wishes to limit the magnitude of an overcurrent by driving it down to an unnatural current-zero.

AC currents can be interrupted at a natural current-zero, which is primarily determined by the circuit alone and practically unaffected by the presence of the interruption device.  Alternately, AC currents can be interrupted at forced current-zeros, which are imposed by the action of the interruption device.  Figure 1.3 illustrates these concepts of natural and forced current-zeros in an AC circuit.

All mechanical switches and mechanical circuit interrupting devices depend on the rapid cooling of the arc medium to open an electrical circuit.  Solid-state switches do not need an arc to break a circuit, since they supply their own conducting medium, the semiconductor material itself.  A semiconductor can conduct current only as long as mobile carriers (electrons and holes) are provided from supply or injection regions within the device.  If the injection of mobile carriers in a semiconductor switch is turned off, then the semiconductor material will revert to an insulating state and block the flow of current – that is, the semiconductor switch will turn off.

The allowable current density within a semiconductor switch is much lower than that which can safely flow in a metal contact/arc switch.  Thus, the cross-sectional size of a semiconductor switch, for equal rating devices, will always be larger than that of a mechanical switch.  Even with this disadvantage, the ease with which a semiconductor switch can be controlled, and the reliability of a device with no mechanically moving parts, portends a bright future for solid-state power switches and circuit breakers.

Overcurrents and protective devices are not new subjects.  Soon after Volta constructed his first electrochemical cell, or Faraday spun his first disk generator, someone else graciously supplied these inventors with their first short circuit loads.  Patents on mechanical circuit-breaking devices go back to the late 1800’s and the concept of a fuse goes all the way back to the first undersized wire that connected a generator to a load.

In a practical sense, we can say that no advance in electrical science can proceed without a corresponding advance in protection science.  An electric utility company would never connect a new generator, a new transformer, or a new electrical load to a circuit that cannot automatically open by means of a protective device.  Similarly, a design engineer should never design a new electronic power supply that does not automatically protect its solid-state power components in case of a shorted output.  Protection from overcurrent damage must be inherent to any new development in electrical apparatus.  Anything less leaves the apparatus or circuit susceptible to damage or total destruction within a relatively short time.  

Examples of overcurrent protection devices are many:  fuses, electromechanical circuit breakers, and solid state power switches.  They are utilized in every conceivable electrical system where there is the possibility of overcurrent damage.  As a simple example, consider the typical industrial laboratory electrical system shown in Figure 1.1.  We show a one-line diagram of the radial distribution of electrical energy, starting from the utility distribution substation, going through the industrial plant, and ending in a small laboratory personal computer.  The system is said to be radial since all branch circuits, including the utility branch circuits, radiate from central tie points.  There is only a single feed line for each circuit.  There are other network type distribution systems for utilities, where some feed lines are paralleled.  But the radial system is the most common and the simplest to protect.

Overcurrent protection is seen to be a series connection of cascading current-interrupting devices.  Starting from the load end, we have a dual-element or slow-blow fuse at the input of the power supply to the personal computer.  This fuse will open the 120 volt circuit for any large fault within the computer.  The large inrush current that occurs for a very short time when the computer is first turned on is masked by the slow element within the fuse.  Very large fault currents are detected and cleared by the fast element within the fuse.  

Protection against excess load at the plug strip, is provided by the thermal circuit breaker within the plug strip.  The thermal circuit breaker depends on differential expansion of dissimilar metals, which forces the mechanical opening of electrical contacts.  

The 120 volt single-phase branch circuit, within the laboratory which supplies the plug strip, has its own branch breaker in the laboratory’s main breaker box or panel board.  This branch breaker is a combination thermal and magnetic or thermal-mag breaker.  It has a bi-metallic element which, when heated by an overcurrent, will trip the device.  It also has a magnetic-assist winding which, by a solenoid type effect, speeds the response under heavy fault currents.

All of the branch circuits on a given phase of the laboratory’s 3-phase system join within the main breaker box and pass through the main circuit breaker of that phase, which is also a thermal magnetic unit.  This main breaker is purely for back up protection.  If, for any reason, a branch circuit breaker fails to interrupt overcurrents on that particular phase within the laboratory wiring, the main breaker will open a short time after the branch breaker should have opened.

Back-up is an important function in overload protection.  In a purely radial system, such as the laboratory system in Figure 1.1, we can easily see the cascade action in which each overcurrent protection device backs up the devices downstream from it.  If the computer power supply fuse fails to function properly, then the plug strip thermal breaker will respond, after a certain coordination delay.  If it should also fail, then the branch breaker should back them both up, again after a certain coordination delay.  This coordination delay is needed by the back-up device to give the primary protection device – the device which is electrically closest to the overload or fault – a chance to respond first.  The coordination delay is the principal means by which a back-up system is selective in its protection.

Selectivity is the property of a protection system by which only the minimum amount of system functions are disconnected in order to alleviate an overcurrent situation.  A power delivery system which is selectively protected will be far more reliable than one which is not.

For example, in the laboratory system of Figure 1.1, a short within the computer power cord should be attended to only by the thermal breaker in the plug strip.  All other loads on the branch circuit, as well as the remaining loads within the laboratory, should continue to be served.  Even if the breaker within the plug strip fails to respond to the fault within the computer power cord, and the branch breaker in the main breaker box, is forced into interruptive action, only that particular branch circuit is de-energized.  Loads on the other branch circuits within the laboratory still continue to be served.  In order for a fault within the computer power cord to cause a total blackout within the laboratory, two series-connected breakers would have to fail simultaneously – the probability of which is extremely small.

The ability of a particular overcurrent protection device to interrupt a given level of overcurrent depends on the device sensitivity.  In general, all overcurrent protection devices, no matter the type or principles of operation, respond faster when the levels of overcurrent are higher.

Coordination of overcurrent protection requires that application engineers have detailed knowledge of the total range of response for particular protection devices.  This information is contained in the “trip time vs. current curves,” commonly referred to as the trip curves.  A trip time-current curve displays the range of, and the times of response for, the currents for which the device will interrupt current flow at a given level of circuit voltage.  For example, the time current curves for the protection devices in our laboratory example are shown superimposed in Figure 1.2.

The rated current for a device is the highest steady-state current level at which the device will not trip for a given ambient temperature.  The steady-state trip current is referred to as the ultimate trip current.  The ratings for the dual-element fuse in the computer power supply, the plug strip thermal breaker, the branch circuit thermal-magnetic breaker and the main circuit thermal-magnetic breaker are 2, 15, 20, and 100 amps, respectively.  Note that, except for the fuse curve, each time-current curve is shown as a shaded area, representing the range of response for each device.  Manufacturing tolerances and material property inconsistencies are responsible for these banded sets of responses.  Trip time-current information for small fuses is usually represented in a single-value average melting time curve.

Even with a finite width to the time-current curves, we can easily see the selectivity/coordination between the different protection devices.  For any given steady-state level of overcurrent, we read up the trip time-current plot, at that level of current, to determine the order of response.

Consider the following three examples for the laboratory wiring, plug strip, and computer system.  

Example 1: Component failure within the computer power supply:  Assume that a power component within the computer power supply has failed – say two legs of the bridge power rectifier – and that the resulting fault current within the supply, limited by a surge resister, is 70 amps.

We see from the fuse trip curve that it should clear this level of current in approximately 20 milliseconds.  If the fuse fails to interrupt the current – or worse, if the fuse has been replaced with a permanent short circuit by a gambling repairperson – the thermal breaker in the plug strip should open the circuit within 0.6 to 3.5 seconds.  The branch thermal-magnetic breaker will open the entire branch circuit within 3.5 to 7.0 seconds, should the plug strip thermal breaker also fail to respond.  Note that no back-up is provided for this particular fault after the branch circuit breaker.  The main laboratory 100 amp thermal-magnetic unit would respond only if the other loads within the entire laboratory totaled greater than 30 amps at the time of the 70 amp power supply fault.

Example 2:  Plug strip overload:  Assume that the computer operator has spilled a drink, and to dry up the mess plugs two 1500 watt hair dryers into the plug strip.  The operator then flips them both on simultaneously, drawing a total plug strip load current of approximately 30 amps.

From the thermal breaker trip curve, we see that the plug strip unit should clear this overload within 5 to 30 seconds.  Note the similarity between the trip curves of the plug strip thermal unit and the branch circuit thermal-magnetic unit in the region of 100 amps and below.  This is because, for these levels of currents, the thermal portion of the detection mechanism within the thermal-magnetic branch breaker is dominant. 

Example 3:  Short circuit within the computer power cord:  Assume a frayed line cord finally shorts during some mechanical movement.  Assume also that there is enough resistance within the circuit, plug strip, and line cord system to limit the resulting fault current to 300 amps.  This level of current is 2000% (20 times) of the rated current of the plug strip thermal breaker, and is beyond the normal range of published trip time specifications for thermal breakers (100% to 1000% of rated current).  Thus the exact trip time range of the thermal unit is indeterminate.

At high levels of fault current, greater than 150 amps in this case, we can see the inherent speed advantage of magnetic detection of overcurrents.  This is evidenced by the fact that the response curve for the thermal-magnetic branch circuit breaker knees downward sharply at current levels between 150 and 200 amps.  At these and higher currents, the magnetic detection mechanism within the thermal-magnetic unit is dominant.  The response curve for the unit crosses over the plug strip thermal breaker response curve (assuming that it extends past its 1000% limit), and coordination between the two interrupters is lost.  The range of response for the thermal-magnetic breaker at 300 amps is 8 to 185 milliseconds.  Should both the plug strip breaker and the branch circuit breaker fail to operate, the main laboratory breaker should clear the fault within 11 to 40 seconds.

If you’re looking to source thermal circuit protection, look no further.  MP’s 40+ years of excellent reputation in thermal breakers is unmatched in the industry, and offers protection from ½ to 70 amperes, in Push to Reset and Switchable versions.  All are ROHS compliant with a broad range of Worldwide accepted agency approvals.  Standard variations or custom requirements are offered for a variety of applications in the Construction, Marine, Medical, RV, Power Generation, Appliance, Floor Care and specialty applications.  Competitive pricing with volume discounts are available for these high quality thermal circuit protectors.  So don’t settle for just any circuit breaker.  Ask for Mechanical Products, a market leader in thermal circuit protection.   Contact Mechanical Products at helpme@mechprod.com.

For decades, MP has been the supplier of choice for the majority of Industrial Floor Care Manufacturers throughout the world.  We believe this is no coincidence, as MP is typically the circuit breaker of choice in demanding applications – and the protection of Industrial Floor Care is one of the most demanding.  Typically in floor care applications, surety of protection against smoke and fire must be provided under harsh environmental conditions where excessive amounts of moisture, solvents, shock, and vibrations are present.  MP breakers excel where additional circuit protection is sought under conditions of extraordinary user-created equipment/circuit stresses/abuse, such as locked motor rotor conditions and electrical current fluctuations created by excessive runs of electrical cords.  Throughout the decades, MP has consistently provided reliable, durable, cost-effective engineered solutions that have helped Industrial Floor Care products successfully perform under these difficult conditions.   For additional information, contact Mechanical Products at helpme@mechprod.com.

The current in thermal and magnetic circuit breakers passes through both a detection mechanism and a set of electrical contacts.  The contacts are generally spring-loaded and latch restrained.  When triggered by the overcurrent detection mechanism, the latch will release a movable contact arm.  The arm then withdraws from the fixed contact at a rate determined by spring loading and electromagnetic forces due to the contact current.

When the contacts are closed, or “latched”, current flows between them only at very small physical contact points, due to roughness on the surfaces of the contact faces.  The actual area of electrical contact is only a small fraction of the facing surfaces of the contacted pair, typically < 1%.  Current flowing in the contacts is constricted at these contact points, much like fluid flowing through a pipe with an insert containing very small holes.  The resistance created by these contact “spots” is referred to as the contact resistance.  The voltage drop across this resistance is then commonly referred to as the contact drop, which in most cases does not exceed more than 0.1-0.2 volts.

Our next article will examine the Parting Dynamics of a pair of contacts when the circuit breaker switches to the open position.

Earlier this year Cooper Bussmann announced the discontinuation of their 174 series Flat-Pak circuit breakers.  If you are looking for a replacement, Mechanical Products Company may have a suitable product to fit your needs.  Please visit us at www.MechProd.com, or contact us at 630-953-4100, and we would be happy to help you find a Mechanical Products part number for your application.

Over the course of the next several upcoming articles, we will present the basics of the behavior of the contact mechanism and the physics of the arc which is present in all electromechanical overcurrent protection devices.

Our discussion of contacts will consider both the electrical and mechanical characteristics of contacts in circuit breaker mechanisms.

Following our review of contacts, we will discuss the development of a simple dynamic thermal model which we will use to consider arc behavior within AC and DC electrical circuits.  

In this context, our next article will consider contact resistance and contact parting dynamics.

Circuit protection devices, like most items involving product or public safety, are regulated and/or tested by some agency.  This is done to confirm compliance with industry or legislative standards.  The agency, depending on the country involved, may be a government department or an independent organization which may or may not have close ties to the government.  While there are several independent laboratories in the United States, the most dominant for circuit protection devices (circuit breakers and supplementary protectors) in the commercial and industrial marketplace is Underwriters Laboratories, Inc. (UL).  Headquartered in Northbrook, IL, UL works closely with several government agencies to establish standards to assure product and public safety.

With origins dating back to 1893, UL now maintains more than 1,000 standards for safety.  Test labs are located throughout the country, with product safety reviewed under major categories such as Electrical; Burglary Protection & Signaling; Casualty & Chemical Hazards; Fire Protection; Heating, Air Conditioning & Refrigeration; and Marine.  Under these standards, UL’s engineering investigations and studies are carried out following strictly defined procedures.  The standards are usually developed through joint agency/industry committees, and submitted to the industry for review and comment prior to adoption.  Depending on the complexity of the standard, several iterations of this process, and a period of several years may occur before a standard is adopted.

For more information on Underwriters Laboratories go to www.ul.com 

Thermal overcurrent protection devices are a versatile, space saving and economical circuit protection option available to the application engineer – a.k.a. thermal circuit breakers and supplementary protectors.  Thermal circuit breakers and supplementary protectors are offered in a wide variety of standard “off the shelf” specified/qualified VAC and VDC ratings ranging from tenths of an Amp to more than 200 Amps.  And, for more unique/challenging applications, certain circuit breaker manufacturers will work closely with the OEM to provide engineered solutions to customize an overcurrent protection device specific to the application requirements.

Thermal circuit breakers are available in a variety of packages, bezel mounts (including snap in) and actuator configurations.  They are available in automatic reset configurations and in a wide selection of switch and/or indicator configurations including rockers, push/pull, toggle, illuminated and push to reset.  The usual number of poles are up to 2.  Terminals can be wires, quick connect blades, screw terminals, Edison base plug type, or fuse clip terminals.  Shunt trip, relay trip and alarm circuit devices are also available. 

Additional design flexibility is available because many thermal circuit breakers are offered compliant to a wide variety of agencies and requirements such as UL489, UL1077, UL1500, CCC, CSA 22.2 No.235-04, IEC, ABYC, SAE J553 and SAE J1625.   

Given the range of options offered in thermal overcurrent protectors, these robust, space saving, economical devices should warrant serious design consideration for the protection of numerous low voltage electrical circuits.

An application engineer needs all of the facts to make an intelligent choice of an overcurrent protection device for a particular application.

The following link is to an article that suggests a seven step procedure that may be followed when selecting an overcurrent supplementary protector, circuit breaker or fuse.

http://www.mechprod.com/overcurrent-protection-selection/

The first six steps in this procedure define the problem in a detailed engineering sense. Then the seventh step, the actual choice of a particular overcurrent protector, can be made from a logical and relevant knowledge base.