Piston rings and their functions.

A piston ring is a seal that surrounds the piston as it moves inside an engine's cylinder wall. Piston rings serve three basic functions: provide a gas-tight seal between the combustion chamber and the crankcase; transfer heat from the piston to cooled cylinder wall; and control oil consumption.

Usually, there are three piston rings for each piston: 

01. The top ring is the top compression ring acts as the primary seal between the piston and the cylinder wall.

 

02. The middle ring acts as a secondary seal between the piston & cylinder wall to help transfer heat to the cylinder wall and assure that no gas escapes from combustion chamber .

03. The bottom the oil control ring is the final ring made up of two smaller rings with a sort of spring separating them. This spring presses the ring against the cylinder wall to protect extra oil consumption.

Circuit breaker selection factors.

 Circuit Breaker selection factors as follows:

01.  Voltage Rating.

02.  Frequency : Circuit breakers up to 600 amps can be applied to frequencies of 50-120 Hz. Higher than 120 Hz frequencies will end up with the breaker having to derate. During higher frequency projects, the eddy currents and iron losses causes greater heating within the thermal trip components thus requiring the breaker to be derated or specifically calibrated. 

03.  Maximum Interrupting Capacity or breaking capacity: The interrupting rating is generally accepted as the highest amount of fault current the breaker can interrupt without causing system failure to itself. Determining the maximum amount of fault current supplied by a system can be calculated at any given time. 

04.  Continuous Current Rating: This ampere rating is the continuous current of the breaker will carry in the ambient temperature(40 deg.) where it was calibrated.

05.  Operating Environment : High ambient temperature and altitude(>4000 Ft) caused derated circuit breaker. At altitude, the thinner air does not conduct heat away from the current carrying components as well as denser air found in lower altitudes. In addition to overheating, the thinner air also prevents the of building a dielectric charge fast enough to withstand the same voltage levels that occur at normal atmospheric pressure.

 The National Electric Code (NEC) determines that circuit breakers should handle 80% of their rated capacity for continuous loads (those being on for three or more hours) and 100% for intermittent loads. For a total current of 15 A, the calculation is as follows: 15 A * 1.25 = 18.75 A. The installer should use a 20 A CB because the immediately lower CB rating, 15 A, would not be enough for this load.

So,a thumb rule is first of all calculate total max. wattage value, then divide by system voltage to get maximum ampere rating. Now just multiply the ampere value by 1.25 to get circuit breaker rating.

Fundamental Characteristics of Circuit Breaker.

Fundamental characteristics of a circuit breaker are:

1. Its rated voltage Ue
2. Its rated current In
3. Tripping current level adjustment ranges for overload protection (Ir or Irth) and for short circuit protection (Im)
4. Its short circuit current breaking rating (Icu for industrial CBs; Icn for domestictype CBs).

Rated operational voltage (Ue)

This is the voltage at which the circuit breaker has been designed to operate.

Rated current (In)

This is the maximum value of current that a circuit breaker, fitted with a specified over current tripping relay, can carry indefinitely at an ambient temperature .If ambient temperature increase circuit breaker will be derated.

Overload & Short circuit relay trip-current setting (Ir & Im)

Short circuit tripping relays (instantaneous or slightly time-delayed) are intended to trip the circuit breaker rapidly on the occurrence of high values of fault current.

Figure (left) - Performance curve of a circuit breaker thermal-magnetic protective scheme; Figure (right) - Performance curve of a circuit breaker electronic protective scheme

Where

• Ir: Overload (thermal or long-delay) relay trip-current setting

• Im: Short circuit (magnetic or short-delay) relay trip-current setting

• Ii: Short circuit instantaneous relay trip-current setting.

• Icu: Breaking capacity

Rated short circuit breaking capacity (Icu or Icn)

The short circuit current-breaking rating of a CB is the highest value of current that the CB is capable of breaking without being damaged. The value of current quoted in the standards is the rms value of the AC component of the fault current, normally given in kA rms

Why Thermal & Magnetic trip units both in Circuit Breaker ?

The thermal trip unit protects against a continuous overload. The thermal unit is comprised of a bimetal element located behind the circuit breaker trip bar and is part of the breaker’s current carrying path. When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts.

The Magnetic trip unit protects against a short circuit. The magnetic trip unit is comprised of an electromagnet and an armature. When there is a short circuit, a high magnitude of current passes through the coils creating a magnetic field that attracts the movable armature towards the fixed armature. The hammer trip is pushed against the movable contact and the contacts are opened. The opening of the breakers contacts during a short circuit is complete in 0.5 milli-seconds.

So,both of the tripping units are necessary for a circuit breaker regarding complete protection.