Breaking Capacity

What is the service breaking capacity and what is the advantage of that on the reliability of breaker?

The rated service short-circuit breaking capacity (Ics) 

In a correctly designed installation, a circuit-breaker is never required to operate at its maximum breaking current Icu. For this reason a new characteristic Ics has been introduced. 

It is expressed in IEC 60947-2 as a percentage of Icu (25, 50, 75, 100%) 

The rated breaking capacity (Icu) or (Icn) is the maximum fault-current a circuit-breaker can successfully interrupt without being damaged. The probability of such a current occurring is extremely low, and in normal circumstances the fault-currents are considerably less than the rated breaking capacity (Icu) of the CB. On the other hand it is important that high currents (of low probability) be interrupted under good conditions, so that the CB is immediately available for re-closure, after the faulty circuit has been repaired. It is for these reasons that a new characteristic (Ics) has been created, expressed as a percentage of Icu, viz: 25, 50, 75, 100% for industrial circuit-breakers. The standard test sequence is as follows: 

O - CO - CO (at Ics) 

Tests carried out following this sequence are intended to verify that the CB is in a good state and available for normal service 

For domestic CBs, Ics = k Icn. The factor k values are given in IEC 60898 table XIV. 

In Europe it is the industrial practice to use a k factor of 100% so that Ics = Icu. 

MAKING CAPACITYOF BREAKER

MAKING CAPACITY

The capacity of a breaker to make current depends upon its ability to withstand and close successfully against the effects of electromagnetic forces. These forces are proportional to the square of maximum instantaneous current on closing. So making capacity is stated in terms of a peak value of current. The peak value of current during the first cycle of current wave after the closure of circuit breaker is known as making capacity. To find making capacity multiply symmetrical breaking current by root 2 to convert from r.m.s to peak and then by 1.8 to include the doubling effect of maximum asymmetry. Making capacity = 2.55 * symmetrical breaking capacity.

BREAKING CAPACITY OF BREAKER

 BREAKING CAPACITY

It is current that a circuit breaker is capable of breaking at a given recovery voltage under specified conditions. The breaking capacity is always stated at the r.m.s value of fault current at the instant of contact separation. When a fault occurs there is considerable asymmetry in the fault current due to presence of d.c component. The d.c component dies away rapidly.

Breaking capacity is expressed in MVA by taking into account the rated breaking current and rated service voltage. Thus if I is the rated breaking current in amperes and V is rated service line voltage in volts, then for a three phase circuit breaking capacity = √ 3 * V * I * 10 –6 MVA.

SHORT TIME RATING OF BREAKER

SHORT TIME RATING

It is the period for which the circuit breaker is able to carry fault current while remaining closed. The fault on the system of very temporary nature persist for 1 or 2 sec after which the fault will be cleared, so the breaker should not be tripped in such situations. This means the circuit breakers should be able to carry high current safely for some specified period while remaining closed. i.e they should have short time rating. It depends on its ability to withstand electromagnetic force effects and temperature rise.

Breaking, Making and Short Time Capacity of Circuit Breaker

Circuit breaker is required to perform the following three duties:

1. It must be capable of opening the faulty circuit and breaking the fault current.

2. It must be capable of being closed on to a fault.

3.  Must be capable of carrying fault current for a short time while another breaker is clearing the fault. 

Depending on the above duties circuit breaker has three ratings breaking capacity, making capacity and short time capacity.

 BREAKING CAPACITY

It is current that a circuit breaker is capable of breaking at a given recovery voltage under specified conditions. The breaking capacity is always stated at the r.m.s value of fault current at the instant of contact separation. When a fault occurs there is considerable asymmetry in the fault current due to presence of d.c component. The d.c component dies away rapidly.

Breaking capacity is expressed in MVA by taking into account the rated breaking current and rated service voltage. Thus if I is the rated breaking current in amperes and V is rated service line voltage in volts, then for a three phase circuit breaking capacity = √ 3 * V * I * 10 –6 MVA.

MAKING CAPACITY

The capacity of a breaker to make current depends upon its ability to withstand and close successfully against the effects of electromagnetic forces. These forces are proportional to the square of maximum instantaneous current on closing. So making capacity is stated in terms of a peak value of current. The peak value of current during the first cycle of current wave after the closure of circuit breaker is known as making capacity. To find making capacity multiply symmetrical breaking current by root 2 to convert from r.m.s to peak and then by 1.8 to include the doubling effect of maximum asymmetry. Making capacity = 2.55 * symmetrical breaking capacity.

SHORT TIME RATING

It is the period for which the circuit breaker is able to carry fault current while remaining closed. The fault on the system of very temporary nature persist for 1 or 2 sec after which the fault will be cleared, so the breaker should not be tripped in such situations. This means the circuit breakers should be able to carry high current safely for some specified period while remaining closed. i.e they should have short time rating. It depends on its ability to withstand electromagnetic force effects and temperature rise.

How Does a Humidistat Work?

A humidistat is a device used to measure and control relative humidity. It can be set for a desired humidity level, with the humidistat signaling to the humidifer to turn off the water supply once that level is attained. Humidistats, in the home, are frequently part of an air conditioning or central heating system and are usually found near the wall-mounted thermostat. While the main purpose of a humidistat-controlled environment is to achieve a level of comfort, they can also be effective in preventing indoor mold outbreaks during particularly hot, moist weather.

What It Is

How It Works (General Terms)

When used in conjunction with an air-conditioning system, a humdistat will "cycle" the conditioner on and off in response to internal humidity level instead of responding to internal temperature levels. Progress-Energy.com provides an example of how a humidistat set at 70 percent relative humidity operates, if it's functioning properly. When the relative indoor humidity reaches 70 percent or above, the air conditioner will "cycle" on, even if the thermostat setting does not call for cooling.

How It Works (Technical Terms)

A typical household humidistat includes a sensing element, made of a material that is sensitive to air moisture, and a relay amplifier. Increases or decreases in indoor humidity strengthen or weaken the electrical resistance occurring between the metal conductors of the sensing element. These variations are in turn gauged by the relay amplifier.

Static Contact Resistance Measurement

The modern High Voltage SF6 circuit breakers have two parallel contact sets. The main contacts are low resistance contacts, which are silver plated, whereas arcing contacts are of tungsten-copper which helps in initiating arc quenching and current interruption. Measurement of the static contact resistance with the breaker in closed position gives the resistance of main contacts only because the arcing contacts are bypassed. Healthiness of main contacts only gets checked during this test. The minimum dc test current should be used according to manufactures specification; however, the IEC and ANSI recommended levels are: 50 A IEC 60694 and 100 A ANSI. 

Static contact resistance is measured by injecting a dc current through the breaker and measuring the milli-voltage drop. A four wire measurement method is used. The breaker must be in the closed position. If low resistance readings are obtained when testing the breaker contact resistance using a low current, then it is recommended to re-test the contacts at a higher current. A higher current will have the ability to overcome connection issues and oxidation on terminals, where a lower current may produce higher readings under these conditions

Dynamic Contact Resistance Measurement

High voltage circuit breakers are extremely important for the function of modern electric power supply systems. The need to predict the proper function of circuit breaker grew over the years as the transmission networks expanded. The maintenance of circuit breakers deserves special consideration because of their importance for routine switching and for protection of other equipments. Electric transmission system breakups and equipment destruction can occur if a circuit breaker fails to operate because of a lack of preventive maintenance. Dynamic Contact Resistance Measurement (DCRM) is known as an effective technique for assessing the condition of power circuit breakers contacts and operating mechanism.