MAKING 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.

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 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.

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 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.

How Does a Humidistat Work?

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.

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.