1.1  The electric network

Electricity is a natural phenomenon. It is a part of matter as negatively charged electrons and positively charged protons.

Generating stations, usually located in remote areas far from the main centers of consumption, generates it by transforming other forms of energy (mechanical, thermal, solar, nuclear, etc.).

Electricity thus produced is transported via the power transmission lines and distributed to consumers via the distribution grid.

The electrical power must always be available, without interruption, to the consumer, even if the consumer does not use it continuously.

The power utilities divide their networks into two broad categories:

1. The transport network
2. The distribution network

At the output of the generating stations, transforming stations step up the production-level voltage to the high voltage necessary to efficiently carry the electricity over longer distances.

The power transmission lines are made of conductors such as overhead lines or underground cables. In spite of their apparent simplicity, these conductors conceal important influencing factors to the electricity transmission network.
 

 1.2  Power circuit interrupters

A power network needs to constantly change the circuits' configurations. That means to put in or out of service this or that part of the installation. Power circuit interrupters include a number of apparatus with the main task to connect and disconnect power circuits rated 1000 Volts and higher.

Main circuit interrupters are:

a) Disconnect-switch:

It is mainly used to isolate equipment or portion of a circuit for repair or maintenance.

The disconnect switch has little or no current interrupting abilities. Its operation has to be done without any current flow in the circuit.

It is a safety device with, usually, a visible breaking contact and it can be locked in the open position.

b) Interrupter:

It is a switch designed to close or open circuits and make or interrupt nominal currents. It is faster than a disconnect-switch and has current breaking capability.

c) Circuit-breaker:

The circuit breaker fills the same function as an interrupter but has the ability to interrupt short circuit currents as well. It is the ultimate protection device on the power network.
 

 1.3  Maintenance strategy

Apparatus on power circuits have to be robust and reliable, otherwise damage can be great on both hardware and personnel.

Maintenance is crucial. Different maintenance strategies exist. The most common are the following:
 

 1.3.1  Corrective maintenance

It consists of intervening only on faults and only to correct and repair.

This strategy has the advantage of spending only on service and parts needed in real time. It also avoids unnecessary spending on periodic maintenance and testing.

On the other hand, the consequences of faulty interrupting equipment may disastrous and very costly in terms of power interruptions to consumers and the extent of damage to the equipment and surroundings, not to mention human safety.

The gains expected on maintenance will quickly fade relative to the high cost of repair and loss of revenue due to power loss.
 

 1.3.2  Periodic maintenance

It is a series of predetermined actions executed periodically, independently of the condition of the equipment.

This method, if applied strictly, can cause a great deal of unnecessary work and increased costss.
 

 1.3.3  Preventive maintenance

It is based on maintenance relying on the actual condition of the equipment. In order to state the condition of the equipment, extensive testing and statistical analyses are conducted periodically and based on experience and new technologies (computers, communications, monitoring, etc.), corrective interventions are planned.

Preventive maintenance is presently the most popular technique for maintenance management personnel in most of the utilities in North America.

Experience has demonstrated that the real life span of the electrical equipment is higher than that estimated by the manufacturers, and equipment in good condition can continue to serve instead of being replaced.
 

 1.4  Circuit breaker testing

Every circuit breaker is factory tested (routine testing) before delivery, tested again on site after installation (commissioning tests), and periodically after that until the end of its life.

These tests are necessary to determine the real condition of the circuit breaker before starting service, to be able to establish a starting point to trace its evolution. One of the most important tests among these is the timing test, and it includes:

- Measuring the exact instant that the contacts change states

- Verify the contacts' discrepancy

- Verify the contacts' travel and speed

The measured values are compared with the established tolerance's limits. Most often, the commissioning test values are used as reference values.

Any deviation from these values can indicate, with the right analysis, what course of action is to be taken.

Before going deeper in discussing timing tests, we need to understand the circuit breaker first.
 

CONCLUSION

An accurate analysis makes it possible to make decisions that are profitable to the breaker, the network and to the maintenance personnel. In order to achieve this, knowing the timing machine and the significance of the operating times is important but not enough.

Knowing well the breaker itself, the reference values (timing chart) and the network characteristics is necessary.

All of this backed with the experience and sense of judgment of the testing personnel.