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.

Low voltage (600 Volts and below) overcurrent protection is achieved through the use of circuit interruption devices which both detect and interrupt the flow of circuit overcurrents.

While solid-state “black box” protection may be uniquely configured to protect an electrical circuit, generally available standard low voltage protection devices are fuses and circuit breakers, in the form of: thermal circuit breakers, magnetic circuit breakers and thermal-magnetic circuit breakers.  The following article link is offered to assist one’s understanding of the differences in these devices.

http://www.mechprod.com/circuit-protection---types/

The article provides a good basic overview of the benefits and shortcomings in using each type of device.  Cost, size, and application benefits and limitations are among the considered attributes.

There is a great deal of difference in the types of overcurrent circuit protectors available to eliminate electrical circuit overloads.  To this end, the following article link is offered to assist one in understanding the differences in the requirements and test parameters of UL489 – the standard for Molded-Case Circuit Breakers and UL1077 – the standard for Supplementary Protectors for Use in Electrical Equipment (aka CBEs – Circuit Breakers for Equipment).

http://www.mechprod.com/circuit-breakers---ul-standards/

The article provides a solid background in UL’s qualification requirements for switching and push to reset circuit protectors.  Both trip free and cycling trip free devices are reviewed for such performance characteristics as overload and short-circuit conditions, endurance, resistive and motor start applications, and level of agency oversight.

This means that the “Hold” and “Trip” characteristics defined on manufacturers’ data sheet (also known as the time/current curve) indicate the performance of the supplementary protector when operating at an ambient temperature of 77°F.

For example, at 77°F a 10 Ampere rated breaker will “Hold” 10 amps indefinitely and “Trip” within the specified time windows for given percentages of overload.  At an operating ambient of 77°F, a typical data sheet might indicate:

Thermal circuit breakers react to the effects of heating.  Therefore, it is important to keep in mind that performance characteristics will differ with changes in ambient temperature.

To achieve the desired “Hold” and “Trip” performance (to avoid nuisance tripping on a circuit at elevated ambient temperatures for example) the ampere rating of the thermal circuit breaker may need to differ from what would be used on the circuit at a nominal 77°F.   This adjustment to the selected CBE’s rating is typically determined by multiplying the rating of the circuit breaker that would be required at the nominal 77°F by an Ambient Temperature Correction Factor (aka Derating Factor) to arrive at the correct circuit breaker rating for the given ambient temperature.

For the 10 ampere circuit protector in our example above, a manufacturer’s typical Ambient Temperature Correction Factor table may indicate the following multipliers:

From the table, for an ambient temperature of 122°F, the 10 ampere nominal circuit breaker in our example would be replaced by a 12.5 – 13 ampere rated supplementary protector to avoid nuisance trips (10 x 1.25).

The selection of the correct circuit breaker rating for a given ambient condition is very simple.  Giving this a few minutes of consideration will help assure your circuit is adequately protected.

Continuing our review of Thermal Circuit Breaker terminology, we will now consider Overload Rating, which should not be confused with Short Circuit Interrupt Capacity (planned for future review).

While testing varies slightly from agency to agency, overload tests are basically fifty “on-off” manual or automatic trip cycles, at a defined multiple of the device’s rated current and at a specific power factor. This testing confirms a device’s capability for either general use (on a resistive load) or in a motor starting application (an inductive load).

Specifically under UL1077, the Overload Rating of a supplementary protector is determined by the device manufacturer from the table below:

Test Current for Overload Tests

From the table, it is easy to see that supplementary protectors qualified for use “Across the line motor starting” must be significantly more robust than the “General use” types.  Within UL1077 this is the difference between Overload Ratings of OL1 and OL0, respectively.

A single device may also have a variety of Overload Ratings at various voltages. In other words, one supplementary protector used in many different applications may be rated at OL1 at 125 Volts AC and OL1 at 30 Volts DC, while at the same time having an OL0 rating at 250 Volts AC.

Caution should be taken when comparing data sheets of various supplementary protectors.  Reference to the thermal circuit breaker’s actual Overload Rating is frequently omitted by manufacturers.  Details on this and other UL1077 qualification categories may be found by visiting the UL Online Certification Directory at, http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.html.

Although the above discussion has been primarily focused on UL1077, another widely used North American standard is C22.2 No. 235-04 published by CSA International. Within this standard, Overload is described in much the same terms as in UL1077, including similar Overload Ratings of OL0 and OL1.  Details may be found within the CSA Certified Product Listings at  http://directories.csa-international.org/

Today’s article continues our review of UL1077 overcurrent terminology by explaining the differences between supplementary protectors that are identified as either “Trip Free” or “Trip Free, Cycling”.

According to UL1077:

A “Trip-Free” supplementary protector is, “a protector designed so that the contacts cannot be held in the closed position by the operating means during trip command conditions.”  In other words, a “Trip-Free” device has the ability to automatically open a circuit in a fault situation, even though its actuator is physically held in the “on” position.  This could also be thought of as “Trip-Free, Non-Cycling” (in contrast to the “Trip-Free-Cycling” described below) by UL1077’s requirements, as the contacts should remain in the open position until the actuator is released and reset.

A “Trip-Free, Cycling” supplementary protector (aka, Cycling Trip-Free) is, “a protector designed so that the contacts cannot be held in the closed position by the operating means during trip command conditions, but will reclose when the closing command is maintained.  The protector will continue to close momentarily and repeatedly as long as the close command is maintained by the operating means during trip-command conditions.” In other words, a “Trip-Free, Cycling” device, with its actuator held in the “on” position, will act as an automatic-reset protective device as long as the fault condition is present, cycling repeatedly between opened and closed circuit conditions until such time as the fault is removed or the device fails.

While the mechanisms required of “Trip-Free” devices make them generally more expensive than “Trip-Free, Cycling” types, due to various design issues associated with the end product, one type of mechanism may be preferable over the other.  Despite the designer’s preference, close attention should be paid to the agency standards applicable to the end product.   Dictates within specific standards may mandate the use of one type of operating mechanism over the other.

Minimum Ultimate Trip & Maximum Ultimate Trip

In recent articles, we explained the real, and many times critical, performance differences that exist between overcurrent devices qualified as Supplementary Protectors per UL1077.  As a further aid in the selection of a UL1077 device, over the course of the next several articles, we will define the key terminology common to the industry and agencies.

The comparison of performance data from one device to the next has relevance only if one clearly understands the underlying meaning behind the terminology associated with the “numbers”.  We must therefore first define what quantity the numbers represent.  Once a term is defined, one then may consider the ramifications of the differences between (seemingly) similar devices.

While our focus will be on terminology most commonly used in the thermal overcurrent protection industry, most of the terms are also common to the data sheets and agency requirements for magnetic-hydraulic and thermal-magnetic types of protectors as well.

For thermal overcurrent supplementary protectors, virtually all electrical performance and associated data, unless otherwise noted, is obtained from evaluating the device at 77?F (25?C).

Today we will consider two terms commonly known in the industry as “Minimum Ultimate Trip” and “Maximum Ultimate Trip”.

The “Minimum Ultimate Trip”, sometimes referred to as the “Hold Current”, is the amount of current the device will conduct indefinitely at a specified ambient temperature.

The “Maximum Ultimate Trip”, sometimes referred to as the “Must Trip” or “Tripping Current” (“TC” for UL1077 and CSA qualification purposes), is the current rating at which a device will trip (open the circuit) within a certain period of time at a specified temperature.

So, what real performance differences might we see through a close examination of the “Hold” and “Trip” currents of seemingly similar devices?

Assuming no differences in the ambient temperature at which the devices were evaluated or in the maximum voltage rating of the devices, a “Minimum Ultimate Trip” of 100%-of-rated-current is a straight forward definition with no real underlying comparative issues with which to be concerned.

Such is not the case with the “Must Trip”.  As you may recall, in an earlier article dealing with the differences in UL 1077 performance we reviewed “Tripping Current” (TC).  Under this UL 1077 and CSA category, the manufacturer of the device is allowed to select the TC level at which to test the device.  As part of the overall qualification testing to the standard, once the TC ( “Must Trip”) level is defined, the agency tests the device to verify that it “trips” within a given time (one or two hours, depending on the protector rating).

Under UL 1077, the “Maximum Ultimate Trip” categorized as Tripping Current (TC0 – TC3) defines a protector’s ability to clear a low level (“slow-cook”, potentially long term damaging) fault.

Here, the importance of understanding the underlying meaning of the data may best be explained by comparing the performance differences between a device rated at, say, TC2 and one rated at TC3.

In reviewing all of the different attributes between protective devices; mechanical, dimensional, environmental, agency qualification, electrical performance, etc., one could very easily overlook the importance of the difference between a TC2 and a TC3 rating. In fact, there is a great deal of difference in the protective performance of these two ratings.

As explained in our earlier article, a TC2 rated device is the least regulated in demonstrating protection against low-level overloads, while a TC3 rated device is considered by many to represent the highest measure of protection against these types of “slow-cook” faults.  This is because a TC2 rated device is tested at some level of tripping current greater than 135%, as selected by the manufacturer.  A TC3 rated device however, is standardized to trip at 135% and 200%-of-rated-current.  From this, it is easily seen that a TC2 rated device tested to trip at 175% within one hour offers far less protection than a TC3 rated device that trips at 135% in one hour and within very specific times at 200%-of-rated-current.

In conclusion, when comparing data between overcurrent protective devices, it is important to pay close attention to your design’s need for protection against “slow-cook” faults as measured by the “Maximum Ultimate Trip”, or “Tripping Current”, as defined by UL and CSA.

Our next article will consider the differences between “trip-free” and “cycling trip-free” overcurrent protection devices.

In considering the most critical UL 1077 performance categories, we first reviewed the Tripping Current (TC) and then Overload (OL) performance ratings supplementary protectors may qualify to.

While we consider the TC and OL ratings equally as critical, the third of our examples, Short-circuit (SC) performance rating, may have  the greatest potential for causing application mistakes.  This is because under the SC performance category,  the circuit protector manufacturer is allowed the greatest latitude in how it may qualify its device to UL 1077.

For surety of protection, the protector selected needs to be capable of  clearing the maximum fault potential of the application.  The OEM’s design/application need for the reusability of the protector after clearing a short circuit also must be considered.  Reusability of the circuit protector after clearing a fault can be very beneficial in limiting an OEM’s future expenses for service calls and warranty claims, and could be considered an essential requirement in critical applications, like medical equipment.

In qualifying a device to UL 1077 for Short-circuit (SC) performance, the supplementary protector manufacturer:

• Defines the test’s:
• Maximum AC and/or DC Voltage.
• Short-circuit (kilo-amperes) current.

• Chooses 1 of 7 UL predetermined SC test set-ups that demonstrate increasing degrees of a protector’s design robustness/capability/survivability.
• See table below for further explanation.  The table defines the various SC codes and offers comments on the implications of each.

By UL definition, the short-circuit current rating in kilo-amperes (kA) is followed by a letter and number designating the test conditions and any recalibration following the short-circuit test as follows:

* The “C” prefix designates the short circuit tests were conducted with a backup device, usually a fuse to simulate upstream/cascaded protection that is present in certain applications.

**  The “U” prefix designates that the short circuit tests were conducted without a backup device, clearly demonstrating the protector’s ability to eliminate the defined fault.

From this summary of UL1077 Short circuit (SC) options,  it is clear that the potential exists for significant performance differences between devices carrying the same “UL Mark”.  For a given level of overload, a protector compliant to an SC rating of U3 offers much greater surety of protection and durability than one merely qualified as a C1 device.   Close attention to the OEM design needs relative to  this particular attribute is essential for maximizing the safety and durability of the OEM’s product.  Needless to say, applications assistance from the supplementary protection manufacturer may prove quite helpful in selecting the protector that best suits a design’s requirements.

In considering the most critical UL 1077 performance categories, we first reviewed the UL Tripping Current codes (TC).  The Overload Rating (OL) is another performance category we consider to be very critical in selecting the correct UL 1077 supplementary protector  for an application.

The Overload Rating (OL) designates whether the protector has been tested for general use or for motor starting applications:

• OL0 =  The device was qualified/tested at 1.5 times the ampere rating for general use.
• OL1 = The device was qualified/tested for motor starting applications
• 6 times the ampere rating for AC applications, and/or
• 10 times the ampere rating for DC applications.

In MP’s experience, the mistaken use of “general use” qualified protectors in “motor starting” applications happens all too frequently.   At start-up and during “locked rotor” faults the inrush current can range from five to ten times the steady state current, depending on the motor design.

If the supplementary device is to protect against potential motor faults, it is very important that the UL 1077 device chosen has an OL1 rating at the applicable voltage.  Suffice it to say, protectors are qualified as ”general use” type because they are incapable of meeting the testing demanded of the “motor starting” requirement(s).  Typically, this is because “general use” qualified protectors use inferior  (less expensive) contacts instead of the silver metal oxide types that are required to achieve the best performance.  In a worst case scenario, under a start-up inrush current or severe overload, a  “general use” protector (misapplied) in a motor starting application could result in a very hazardous fire situation.  Under start-up inrush current or a severe overload, the incapable contact mechanism may result in contacts welding together, creating a permanent conductive path with virtually no protection, unless the device reaches a catastrophic state and the protector actually fuses.