Introduction
Safety is critical when conducting no-outage testing and
statements regarding safety are contained at the beginning
of this and all No-Outage Corner articles for that reason.
Detailed safety requirements for work on and around energized
equipment are clearly defined in NFPA 70E and
do not require additional discussion here. Certain types
of activities on and around energized equipment are more
dangerous than others and while proper safety guidelines
must be followed at all times, we must also realize that the
chances of an incident occurring during some activities are
greater than others. Among these more dangerous activities,
switching, racking out circuit breakers, taking voltage measurements,
and removing switchgear covers are some of the
most common and yet most potentially dangerous tasks.
Switching must be conducted prior to any de-energized
maintenance activity, and new technology has allowed this
activity to be performed in a much safer manner. Additionally, new no-outage testing technologies can be combined
conveniently with this and other new safety technologies
to provide valuable and previously unknown equipment
condition information.
Switching
The action of opening a circuit breaker or switch causes the abrupt
interruption of power which is inherently dangerous. The severe electrical
and mechanical stresses created during this event can be influenced
by the equipment condition. An insulation or conductor flaw can impair
optimum interruption which can lead to an arc-flash or arc-blast incident.
The operator must always be aware that an incident may occur during the
act of switching. This is especially unnerving when the operator must stand
directly in front of the equipment when performing manual switching.
Fortunately, a series of new actuators (see Figure 1) are available today
that allows the operator to perform switching from a safe distance from
the equipment. The actuator design varies for different types of equipment.
For typical circuit breakers, a magnetic base allows the actuator to
be temporarily placed on the circuit breaker. The operator can then open
the breaker by standing outside of the exposure zone and trigger the safety
device which depresses the breaker’s manual open button.
Difficulties
In most cases, it is very inconvenient, if not nearly impossible, to de-energize distribution substations in order to conduct traditional outage-based preventive maintenance testing. Often, distribution design does not incorporate redundant substations, and any outage results in a complete power loss to the entire secondary network which may affect multiple facilities. These serious shortcomings exemplify the critical role of distribution substations and bring to light the need for enhanced reliability.
Racking out
The action of removing or installing a circuit breaker from a hot bus
may be as dangerous as switching. The racking action necessitates that
the operator stand directly in front of the breaker. As the breaker is methodically
extracted from the bus, several potential problems may occur.
In addition to typical concealed insulation or conductor flaws, the breaker
fingers historically have been a problem by either falling apart during racking
out or by misalignment problems during racking in. These problems
may result in an incident. Technology has yet again provided an innovative
solution to this dangerous procedure by the development of a universal
remote breaker racking device as shown in Figure 2.
During typical operation, the operator first selects the appropriate socket
that fits the specific breaker racking mechanism and then moves the remote
device in place in front of the breaker. After a few adjustments are made,
the operator can rack out or rack in the breaker from a safe distance.
Electromagnetic Signals
It is well known that medium- and high-voltage insulation that is failing
due to partial discharge and corona activity creates high frequency
electromagnetic signals. Similar signals are also produced by arcing from
severe conductor problems. These electromagnetic signals which are
symptoms of dangerous defects can now be safely detected by placing
a simple hand-held test device against the outside of the switchgear as
shown in Figure 3. This simple check can be made before racking out or
switching off a circuit breaker to detect a high percentage of concealed
problems and after racking in or switching on a circuit breaker to detect
a high percentage of potential problems such as arcing due to improper
breaker alignment.
This test device’s versatility goes well beyond just checking switchgear
for safety purposes. Equipped with three different types of sensors and
a simple multicolored LED display, the test set is capable of performing
basic insulation condition surveys of medium- and high-voltage switchgear,
cable, transformers, bus duct, motors, generators and many other devices.
Due to the recent popularity of this new technology, it will be featured
in a future article.
First Trip
Typically, circuit breakers remain dormant
for very long periods of time, but when called
upon to interrupt a fault, they are expected
to operate within finite times based upon
their protective settings. These settings are
derived by conducting coordination and
arc-flash studies to optimize electrical system
performance and enhance worker safety.
Typically, the breaker’s lubrication condition changes over time, due to a number of factors, and hardens
to a point where the breaker’s first trip is inhibited. This
first trip represents the breaker’s true operating time and
often differs greatly from the values expected in the studies
thus adversely affecting system performance and exposing
the worker to higher levels of incident energy and making
the studies invalid.
The only solution to validate the studies and to protect the
worker is to frequently verify breaker timing under first trip
conditions. Typical maintenance and testing occurs after the
breaker has already been tripped once, and thus the critical
first trip condition information is lost. Regular exercising
of the breaker can help the first trip performance. Frequent
maintenance will also help, but these activities do not ensure
optimum first trip operation.
Breaker Vibration Signature
A new technology has evolved that efficiently determines
breaker first trip performance thus enabling validation of the
arc-flash study. This technology has the additional benefit of
verifying overall breaker mechanical condition.
A special transducer is magnetically coupled to the front
of the breaker as shown in Figure 4. After the technician
moves from the exposure zone, the breaker is operated. The
transducer sends valuable vibration signals that represent
breaker first trip timing and mechanical condition to a handheld
analyzer. This unique breaker vibration signature is
compared to a database library of known signatures. Posttest
analysis can include superimposing the test breaker signature
upon good breaker signatures, as shown in Figure 5, to
quickly spot potential problems. Should the breaker fail the
first trip test in the field, it is necessary to perform complete
shop reconditioning or remanufacturing in order to return
the breaker to a condition that will ensure proper operation
and validate the arc-flash study. Frequent performance of
the breaker vibration signature test ensures performance and
exercises the breaker mechanism. Further details regarding
this new technology will be featured in a future article.
Conclusion
As necessary safety regulations have evolved to better
protect the technician, so has grown the array of technologies
available to provide better and more practical protection
methods. Along with these developments, additional
technologies have been developed to efficiently determine
equipment condition in conjunction with performing the
safety-related tasks. Potentially dangerous electrical work
activities can now be performed with greater safety, and
these activities create an ideal opportunity to obtain critical
equipment condition information.
By adding the following steps to your safety procedures,
safety will be enhanced and valuable equipment condition
information can be obtained. This valuable equipment condition
information can then be utilized to further enhance
safety and reliability.
- Always use appropriate PPE.
- Test the switchgear for abnormal electromagnetic activity
(see figure 3).
- Set remote actuator (see figure 1) in place.
- Set vibration transducer (see figure 4) in place.
- Move out of exposure zone and remotely trip breaker.
- Set remote racking device (see figure 2) in place.
- Move out of exposure zone and remotely rack out breaker.
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