Chapter 1

Chapter 3Identify and inspect Relay, Sensor and Fuse BASIC HYDRAULIC & PNEUMATIC SYSTEM 2CE 2463Prepared by:Pn. Zainon Sharmila ShamsuddinWhat is Relay ?A relay is usually an electromechanical device that is actuated by an electrical current.

The current flowing in one circuit causes the opening or closing of another circuit.

Relays are like remote control switches and are used in many applications because of their relative simplicity,long life, and proven high reliability.

Relays are used in a wide variety of applications throughout industry, such as in telephone exchanges, digital computers and automation systems. What is Relay ?Highly sophisticated relays are utilized to protect electric power systems against trouble and power blackouts as well as to regulate and control the generation and distribution of power.

In the home, relays are used in refrigerators, washing machines and dishwashers, and heating and air-conditioning controls.

Although relays are generally associated with electrical circuitry, there are many other types, such as pneumatic and hydraulic. Input may be electrical and output directly mechanical, or vice versa.

Types of Relays

There are two basic classifications of relays: Electromechanical and Solid State.

Electromechanical relays have moving parts, whereas solid state relays have no moving parts.

Advantages of Electromechanical relays include lower cost, no heat sink is required, multiple poles are available, and they can switch AC or DC with equal ease.Electromechanical Relays

General Purpose Relay: The general-purpose relay is rated by the amount of current its switch contacts can handle. Most versions of the general-purpose relay have one to eight poles and can be single or double throw. These are found in computers, copy machines, and other consumer electronic equipment and appliances.

Power Relay: The power relay is capable of handling larger power loads 10-50 amperes or more. They are usually single-pole or double-pole units.

Contactor: A special type of high power relay, its used mainly to control high voltages and currents in industrial electrical applications. Because of these high power requirements, contactors always have double-make contacts.

Solid State RelaysThese active semiconductor devices use light instead of magnetism to actuate a switch. The light comes from an LED, or light emitting diode.

When control power is applied to the devicesoutput, the light General Purpose Relay is turned on and shines across an open space. On the load side of this space, a part of the device senses the presence of the light, and triggers a solid state switch that either opens or closes the circuit under control.

Often, solid state relays are used where the circuit under control must be protectedfrom the introduction of electrical noises.

Advantages of Solid State Relays include low EMI/RFI, long life, no moving parts, no contact bounce, and fast response. The drawback to using a solid state relay is that it can only accomplish single pole switching.SensorsAsensoris aconverterthat measures aphysical quantityand converts it into a signal which can be read by an observer or by an (today mostlyelectronic) instrument.

A sensor is a device, which responds to an input quantity by generating a functionally related output usually in the form of an electrical or optical signal.

Thermocouple sensor for high temperature measurementFusesInelectronicsandelectrical engineering, afuseis a type of low resistanceresistorthat acts as a sacrificial deviceto provideovercurrentprotection, of either the load or source circuit.

Its essential component is a metal wire or strip that melts when too much current flows, which interrupts thecircuitin which it is connected.

Short circuit, overloading, mismatched loads or device failure are the prime reasons for excessive current.

A fuse is a device that is rated in amps and design for the circuit its being used in. Never install a larger fuse than is recommended is any electrical circuit

Fuse ConstructionA fuse consists of a metal strip or wire fuse element, of small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and (usually) enclosed by a non-combustible housing.

The fuse is arranged in series to carry all the current passing through the protected circuit. The resistance of the element generates heat due to the current flow.

The size and construction of the element is determined so that the heat produced for a normal current does not cause the element to attain a high temperature.

If too high current flows, the element rises to a higher temperature and either directly melts, or else melts a soldered joint within the fuse, opening the circuit.

Fuse ConstructionThe fuse element is made of zinc, copper, silver, aluminum, or alloys to provide stable and predictable characteristics. The fuse ideally would carry its rated current indefinitely, and melt quickly on a small excess. The element must not be damaged by minor harmless surges of current, and must not oxidize or change its behavior after possibly years of service.

The fuse elements may be shaped to increase heating effect. In large fuses, current may be divided between multiple strips of metal. A dual-element fuse may contain a metal strip that melts instantly on a short-circuit, and also contain a low-melting solder joint that responds to long-term overload of low values compared to a short-circuit. Fuse elements may be supported by steel or nichrome wires, so that no strain is placed on the element, but a spring may be included to increase the speed of parting of the element fragments.

The fuse element may be surrounded by air, or by materials intended to speed the quenching of the arc. Silica sand or non-conducting liquids may be used.

Type of FusesLow voltage fusesIn this category all fuses up to 1.5 kV can be included. But the most typical voltage levels for low voltage fuses are 500 V, 690 V and 750 V.

LV HRC fuses are used for installation systems in non-residential, commercial and industrial buildings, as well as in the switchboards of power supply companies. They therefore protect essential building parts and installations.

LV HRC fuse links are available in the following operational classes:gG (previously gL) for cable and line protectionaM for the short-circuit protection of switching devices in motor circuitsgR or aR for the protection of power semiconductorsgS operational class combines cable and line protection with semiconductor protection.Type of Fuses

Type of FusesAll fuses used on power systems from 1.5 kV up to 138 kV are categorized as high voltage fuses. High voltage fuses are used to protect instrument transformers used for electricity metering, or for small power transformers where the expense of a circuit breaker is not warranted. For example, in distribution systems, a power fuse may be used to protect a transformer serving 13 houses. A circuit breaker at 115 kV may cost up to five times as much as a set of power fuses, so the resulting saving can be tens of thousands of dollars. Pole-mounted distribution transformers are nearly always protected by a fusible cutout, which can have the fuse element replaced using live-line maintenance tools.Large power fuses use fusible elements made of silver, copper or tin to provide stable and predictable performance. High voltage expulsion fuses surround the fusible link with gas-evolving substances, such as boric acid. When the fuse blows, heat from the arc causes the boric acid to evolve large volumes of gases. The associated high pressure (often greater than 100 atmospheres) and cooling gases rapidly quench the resulting arc. The hot gases are then explosively expelled out of the end(s) of the fuse. Such fuses can only be used outdoors.Type of Fuses

Type of FusesResettable fusesSo-called self-resetting fuses use a thermoplastic conductive element known as a Polymeric Positive Temperature Coefficient (or PPTC) thermistor that impedes the circuit during an overcurrent condition (by increasing device resistance). The PPTC thermistor is self-resetting in that when current is removed, the device will cool and revert back to low resistance. These devices are often used in aerospace/nuclear applications where replacement is difficult, or on a computer motherboard so that a shorted mouse or keyboard does not cause motherboard damage.

Type of FusesThermal fuse A thermal fuse is often found in consumer equipment such as coffee makers or hair dryers or transformers powering small consumer electronics devices. They contain a fusible, temperature-sensitive alloy which holds a spring contact mechanism normally closed. When the surrounding temperature gets too high, the alloy melts and allows the spring contact mechanism to break the circuit. The device can be used to prevent a fire in a hair dryer for example, by cutting off the power supply to the heater elements when the air flow is interrupted (e.g., the blower motor stops or the air intake becomes accidentally blocked).