Time and travel analysis is the most important test used to determine the correct operation of a circuit breaker.

Although the majority of technicians perform timing measurements; travel analysis, or motion measurements, are often ignored. The motion measurement of the contacts and mechanism in circuit breakers provide a variety of important information when performing routine testing and maintenance; however it is a practice that is marginalized due to its apparent difficulty and complexity.

Bypassing this measurement and its results can lead to a catastrophic failure in the circuit breaker.

A circuit breaker is defined as “A mechanical switching device” and the way to determine if the mechanical switching function is working correctly is to hook up a time and travel analyzer to evaluate the operating characteristics of the circuit breaker.

When bulk oil circuit breakers were the prevalent technology in substations, travel measurements were performed without question. Over the years, because of various reasons such as lack of outage time, lack of maintenance personnel, and the complexity of hooking up a transducer to the circuit breaker, travel measurements are being performed less and less. Time and travel measurements have been replaced with just contact timing or reduced to solely first trip testing.

A recent study by CIGRE working group A3.06 released in 2012 found that 50% of the major failures in a circuit were due to the operating mechanism and 30% were due to the electrical control and auxiliary circuits.

While contact timing and first trip are important tests and should be performed, they will not fully test the operating characteristics of the mechanism; therefore neglecting travel measurements should not be standard practice if the true health of the circuit breaker is to be determined.

Although the transducer can be attached to the circuit breaker to determine the travel of different parts of its components such as the dashpot, the primary application of a motion transducer is to measure the motion of the main/arcing contacts in the circuit breaker.

The transducer can be hooked to many different parts of the circuit breaker, directly to the pull rod of the main contacts, directly to the mechanism, somewhere on the linkage in between or even to an auxiliary switch.

The stroke is defined as the total travel distance of the contacts from resting position in one state, e.g. Closed, to the resting position in the other state i.e. Open or vice versa.

It is imperative that one is diligent in connecting the transducer to the manufacturer’s recommended attachment point on the circuit breaker and correcting for any multiplication factors if a direct connection to the contacts cannot be made.

At the very least the technician must be consistent in measuring the stroke through periodic maintenance in order to trend the results.

If the circuit breaker is gang operated i.e. it has one mechanism operating all three phases then only one transducer is needed, if the circuit breaker has separate operating mechanisms for each phase, then an individual transducer should be used for each mechanism.

Once the stroke of the circuit breaker is determined you can derive the velocity of the contacts in different regions of the travel. The most common region to measure velocity is during the arcing zone of the circuit breaker where it is actually interrupting or clearing the fault.

Occasionally damping is also measured on the travel curve by calculating the velocity in the damping zone or the time between two predefined points on the travel curve in the damping zone.

By observing the closing time along with the travel measurement the penetration or contact wipe can be determined, this is how far the contacts are engaged. Penetration is the length measured from initial contact touch to the final resting position after the operation.

Overtravel is measured directly from the travel curve and is the maximum displacement past the resting position that the contacts reach during the operation.

Rebound is measured from minimum displacement, after the maximum displacement (Overtravel), to the final resting position of the contacts.

There are two types of transducers used to measure the contacts of the circuit breaker, linear or rotary. A linear transducer will measure a value of length typically in either inches or millimeters whereas a rotary transducer will measure a value of angle, typically degrees, which then must be converted to inches or millimeters.

There are various designs of transducers such as resistive, optical, magnetic etc. but they generally give an analog or digital output.

Linear transducers are available in a wide variety of lengths ranging from 25 mm or less, commonly used for vacuum circuit breakers, all the way up to 1000 mm or more, typical lengths are from 225-300 mm for SF6 dead tank circuit breakers and 500-600 mm for bulk oil circuit breakers.

When first setting up for motion measurements, the type of transducer to be used, rotary or linear, must be selected.

Although a lot of breakers allow you to use either type of transducer, the manufacturer generally will specify a preference and it is always recommended to use the manufacturer’s specified attachment point, transducer type, and conversion factor (if needed).

Caution: Before connecting the transducer, always ensure that the circuit breaker is in the open position, make sure no energy is stored in the mechanism, or if it is impractical to discharge all the energy in case of some pneumatic or hydraulic mechanisms, make sure the maintenance pin that blocks operation is set, finally de-energize the power going to the control circuitry. No matter where the transducer is placed, no part of the transducer, mounting bracket, or travel rod if used, must be in the direct path of any moving parts of the circuit breaker that will cause damage to the transducer or its accessories.

If a linear transducer is used it must be of suitable length to cover the total travel distance the transducer will encounter, including overtravel, on both close and open operations.

If unsure of whether the transducer is of suitable length, a common practice is to attach the transducer in one position e.g. closed, then detach the transducer and operate the circuit breaker so it changes state. Now that the circuit breaker is in the opposite position i.e. open, re-attach the circuit breaker to see if it is of suitable length.

Once a transducer of adequate length is selected, the next consideration in size is to make sure the transducer will fit in the space provided.

There are two types of measurements made with a linear transducer, the first is a direct measurement as in Figure 2, where the transducer or linkage rod is connected directly to the moving contacts; direct measurements are common in bulk oil circuit breakers, minimum oil breakers, most vacuum CBs and some dead tank SF6 circuit breakers.

Although finding a suitable mounting bracket and correctly mounting the transducer can be difficult at times, this method is beneficial because the actual stroke of the transducer is equal to the actual stroke of the contacts, therefore no conversion factor is needed and all parameters measured with the transducer are direct representatives of the motion of the contacts in the circuit breaker; the motion isn’t distorted by any gears, linkages or mechanical play of the interconnections.

The second type of measurement with a linear transducer is an indirect measurement, where the transducer is not connected directly to the moving contacts but to a part of the circuit breaker that is connected to the moving contacts such as the mechanism or interconnecting linkage.

When this type of connection is used, the travel of the transducer may or may not be equal to the travel of the main contacts. If the travel of the transducer is different, a conversion factor or ratio should be used in order to obtain the correct stroke length and travel parameters of the circuit breaker.

For example, 80 mm of transducer stroke can be equivalent to 120 mm of contact stroke so a multiplication factor of 1.5 shall be used.

When a rotary transducer is used, the measured quantity is in degrees, or occasionally radians, and is then converted to a unit of length.

There are two types of conversions, the first is a constant conversion where one degree is equal to a certain value of length throughout the entire travel of the contacts. This is common where the mechanical linkage is simple with few interconnecting parts.

When the linkage is more complicated, the ratio of the angle to length may not be constant throughout the total travel of the contacts i.e. one degree might equal one and a half millimeters for the first ten degrees then for the next ten degrees of travel, one degree might equal two and a half millimeters. In these cases a conversion table must be used to account for the varying values.

Rotary transducers have the advantage of being relatively small and with a few accessories one kit can hook up to many different styles or types of circuit breakers.

The disadvantage of the rotary transducer is that you need to know the conversion factor or table in order to calculate the correct parameters of your circuit breaker.

If the conversion factor or table is not provided in the manual, the breaker manufacturer should be contacted.

Rotary transducers are most common on live tank SF6 circuit breakers but are also used on certain types of dead tank SF6 breakers, bulk oil circuit breakers and generator circuit breakers.

Once the proper transducer, and conversion factor if needed, is selected, most of the parameters such as stroke, overtravel, rebound, penetration etc. will come out of the measurements automatically.

One of the exceptions to this is the velocity measurement.

In order to calculate the velocity the analyzer must be told where on the curve velocity should be measured.

Two different points on the travel curve are selected and the average velocity between these two points is calculated.

These points can reference many different points on the curve such as distance below close, distance above open, percentage of stroke, distance below upper point etc.

Another common reference point is an event during the timing e.g. contact touch or contact separation.

The two most important factors that influence the selection of the speed calculation points are that the velocity is measured on a linear portion of the travel curve and that it is measured during the arcing zone, therefore the calculation points are generally near contact touch on the close and contact separation on the open.

A common misconception for calculating velocity is to take the total travel distance (stroke) and divide it by the total time it takes for the contacts to reach the fully closed position, this will not determine the velocity during the arcing zone but the average velocity for the total travel. In this case the acceleration at the beginning of the travel and deceleration at the end will mask the instantaneous velocity around the arcing zone.

A common misconception for calculating velocity is to take the total travel distance (stroke) and divide it by the total time it takes for the contacts to reach the fully closed position, this will not determine the velocity during the arcing zone but the average velocity for the total travel. In this case the acceleration at the beginning of the travel and deceleration at the end will mask the instantaneous velocity around the arcing zone.