MECHANICAL PRODUCTS ANNOUNCES ANOTHER

NEW FAMILY OF CIRCUIT BREAKERS:

THE SERIES 15 PUSH-TO-RESET

Available in Amp ratings from 3 to 40 Amps, the NEW 15 Series offers an economical, compact design in single pole circuit protection.  Offering the same small package size from 3 to 40 amps will make panel installation designs much easier, efficient and cost effective.  All Series 15 Thermal Circuit Breakers have a rated voltage of 125/250VAC, 50 VDC with an Interrupt Capacity of 1000A and a Dielectric Strength of 1500 VAC.  They are ROHS Compliant, UL1077, CSA Approved and UL1500 Ignition Protected. VDE and CCC certification is curently Pending.

The MP 15 Series is designed for various applications:  Marine, Household Appliances, Power Strips, Lab Furniture, Audio Visual Equipment and more.  Qualified for resistive and motor load applications, they are ideal for Marine Panels, Transportation, Floor Care and other motor start up applications.  The 15 Series Thermal Circuit Breakers are available in various threaded and snap-in bushings, along with multiple Quick Connect and Screw Terminal styles to meet nearly every installation requirement. 

MP has been a leading designer and supplier of thermal circuit protection since 1943.  MP circuit breakers are used in thousands of commercial and industrial applications ranging from medical equipment, appliances, lighting and communication devices, to marine, recreational and off road vehicles/equipment, and electrical protection devices.  MP has been management owned and operated since 1998, is headquartered in Lombard, Illinois and maintains manufacturing capabilities in the US and overseas.  For additional information, visit our website at www.mechprod.com.

The device current in thermal and magnetic circuit breakers passes through both a detection mechanism and a set (or sets) 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 the contacts only at very small physical contact points, or asperities, due to surface roughness on the bulk contact faces.  The actual area of electrical contact is only a small fraction, less than 1%, of the apparent area of the bulk contact surface (see Figure 5.1).

Current flowing in the contact bulk regions is constricted at these contact points, much like fluid flowing through a pipe with an insert containing very small holes.  The extra electrical resistance due to this current restriction is referred to as the spreading or constrictive resistance of the contact.  It can be shown [5.1] that the constriction resistance on each side of an individual contact “spot” is given by

where ϱ_r is the bulk resistivity of the contact material, and a is the effective radius of the asperity or actual contact spot area.  If the contacts are constructed of two different materials, with respective bulk resistivities  ϱ_r1 and ϱ_r2, the total series spreading resistance due to current constriction in both contacts is

Normally, contacts are fabricated with identical materials and, normally, actual contact is made at N number of spots on the contact surfaces.  The net constriction resistance for the contacts is then the parallel combination of all the individual contact values, or

The effective radius of each contact spot, a_i, is dependent on the preparation of the bulk contact surface, the normal forces applied to the contacts, the “hardness” of the contact material (i.e will each contact asperity be under elastic or plastic deformation?), and the temperature at the contact interface.

In addition to constrictive resistance at contact asperities, there may be a resistance due to a thin film or layer of material oxide between contacting asperities.  Electrons either tunnel quantum mechanically through this thin film, or break through the film by a process Holm refers to as “fritting” [5.1].  The film resistance is between the constriction resistances of individual asperities, so the net “contact” resistance would be a modification of Equation (5.1):

where R_fi is the film resistance at asperity i.

In practice there is no attempt to determine contributions to R_contact due to individual contact spots.  The net excess resistance of the contact system, beyond the bulk resistances of the two contacting bodies, is simply referred to as the contact resistance.  The voltage drop across this resistance is commonly referred to as the contact drop.  In most cases this contact drop does not exceed .1-.2 volts.  Contact drops tend to saturate at these levels since, as the magnitude of the current rises, the asperity interface temperature rises softening the asperity material.  The softer material spreads out and increases the actual asperity contact area, thus lowering the contact resistance.

When two bulk metallic contacts which are carrying an electrical current separate, the last point or points of physical and electrical contact will be at one or more (if more than one, a small number) constriction asperity spots.  The current density at these points will be very large, easily enough to melt the asperity material and form molten bridges between the two contacts.  These bridges are then heated and stretched to the point that they vaporize.  The process initiates the arc between the two contacts.  If the contacts are not metallic, such as carbon, the asperity points do not melt, but rather arc immediately upon physical separation.  

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.