Eaton Heinemann      Hydraulic-Magnetic Circuit Breakers
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Protection against short circuits or overcurrents which can destroy the equipment or put other nearby or interconnected equipment out of service

TECHNOLOGIES I. USUAL SYSTEMS OF PROTECTION Fuses These consist of a metallic resistance mounted inside an insulated tube, such as glass, or coated with a silicon-sand inside a ceramic holder. The metallic resistance is usually made of a lead alloy which melts due to the heating effect of the electric current and breaks the circuit as a result. Such devices are relatively inexpensive but nevertheless have a number of disadvantages: The point at which the circuit is broken varies with changes in ambient temperature and the tolerance of the fuse can be as high as 50%. It is also necessary to replace the fuse each time that the circuit is broken. They only provide short circuit not overload protection and suffer from thermal stress so occasionaly breaking the circuit when there is not a real faulty situation. Thermal and thermal-magnetic circuit breakers Thermal circuit breakers, or as commonly known thermal cut outs, incorporate a bimetallic element in series with the circuit. When overload current is applied heat is generated which deforms the bimetal and causes the contacts to open. Compared to fuses they have the advantage that they can be reset after eliminating the overload. The main disadvantage is they are too slow to protect in the case of a short circuit or major overload due to the thermal inertia in the bimetal. Therefore it is desirable to ad a magnetic system which instantly trips the breaker on short circuit. One of the major weaknesses of this technology is its dependence on ambient temperature with the consequence of a quite important variation of the tripping point (up to 50%). It is absolutely mandatory to take this temperature parameter into consideration by selecting the nominal current of the circuit breaker. Another weakness to be mentioned compared with the hydraulic-magnetic breakers described later is the relatively high internal resistance generating a voltage drop at the terminals of the circuit breaker. On switch-on all inductive loads, such as transformers, produce a high current peak which will immediatly trip a sensitive circuit breaker. It will not therefore be possible to start the equipment. HYDRAULIC-MAGNETIC CIRCUIT BREAKERS With these protection devices, tripping of the circuit breaker results from changes in the magnetic field of a coil through which the current flows: This is the principle of the electromagnetic relay. The coil is wound on a sealed, non-ferrous tube which is filled with a silicone fluid. Inside there is a movable core held under the tension of a spring. This tube is an integral part of the magnetic circuit and the silicone fluid regulates the movement of the core. The principle of operation is illustrated below.. At normal current or less, the magnetic field is not strong enough to move the core. During a small overload, the reluctance falls and the strength of the magnetic field increases and attracts the core towards the armature. The magnetic flux developed is proportional to the current and becomes sufficient to counteract the effect of the spring. The viscosity of the silicone fluid regulates the motion of the core thereby providing a required time delay, which is inversely proportional to the overload current. When the magnetic flux reaches a predetermined value, the core is attracted towards the pole at a speed which is dependent on the viscosity of the fluid. The reluctance of the magnetic circuit is then greatly reduced, the armature falls on to the pole thereby breaking the circuit. In the event of a large overload or of a short circuit, the magnetic field generated by the coil is in itself sufficient to attract the armature without the position of the core being changed. Tripping is therefore instantaneous. In comparison to thermal-magnetic circuit breakers the must trip point does not change with the ambient temperature. The tolerance (minus 0 % to + 25 %) of the must trip point is considerably closer controlled than fuses or thermal breakers. Furthermore, the hydraulic-magnetic circuit breaker can be re-closed immediately after removal of the fault whereas the thermal circuit breaker can only be re-closed after cooling.                 On the other hand, the time delay is modified as a result of the temperature because of the variation in viscosity of the fluid used. The change is inversely proportional to the temperature, circuit breaking will be faster at high temperature thereby raising safety margins. This variation is insignificant in comparison to these of thermal circuit breakers. The figure shows the most important components of a hydraulic-magnetic circuit breaker. By varying wire diameter and the number of turns in the coil, the circuit breaker's nominal trip current can be determined. In principle, any value between 10 mA and 700 A can be obtained. The fluid viscosity will determine the circuit breaker's response time. It is of course possible to envisage a circuit breaker which trips instantly, in which the fluid will then be replaced by an iron cylinder. Mechanical circuit breaking devices have been developed in such a way that breaking the circuit cannot be prevented from outside the device, and manual re-closure in the event of a short circuit is rendered impossible. The circuit breaker is fitted with a lever which acts as a manually operated switch having just two positions, "open" or "closed". This makes it possible to see if the circuit has been broken or not, without any chance of confusion. As an option the circuit breaker can be proposed with a three position level, "ON", "OFF" and "Mid-Trip" allowing to differentiate an electrical switch "OFF" from a manual switch "OFF". General Characteristics of hydraulic-magnetic Circuit Breakers Nominal Current (In) In, is the current which the circuit breaker must stand over an indefinite period. In excess of 125 % of In, the circuit breaker must open within a specified time delay. Any current is permitted within the extreme limits of each device type. Nominal Voltage (Un) Un is the working voltage across the open-circuited circuit breaker terminals. Time delay Various selections of time delays are offered for most circuit breakers (see catalogues for graphs). In general, they show a graph on which one axis is graduated in percentages of In, and the other axis in seconds. The shortest trip time is limited to a few milliseconds due to inertia in the switching mechanism. For a value corresponding to 125 % of In, a trip delay time of up to three minutes can be achieved. Interrupting capacity It is the highest current that the device can break at the nominal voltage without any damage. Depending of the size of the circuit breaker, interrupting capacities from 1000 A to 25000 A can be achieved. Approvals and applications Catalogues refer to various standards, such as UL, CSA, VDE or SEV, to quote some of the most common. Such standards confirm that certain parameters or essential safety standards, such as dielectric and mechanical strengths, electrical resistance, etc., are met by the manufacturer. Internal circuits The hydraulic-magnetic circuit breakers offer various internal configurations making them ideally suited for a wide range of applications. The principal internal circuits are: Switch Series trip Series trip with auxiliary contact Shunt trip Relay trip Dual-rating DUal CONtrol Series trip for high inrush current II. TWO SPECIAL CIRCUITS In the first part, normal circuit protectors have been presented; two special circuits will be described now: the Ducon and High-Inrush systems. Circuit breakers "DUal CONtrol" The first of these offers an interesting range of applications which results from its two electrically isolated coils which act on a common hydraulic-magnetic circuit breaking system. The first coil is connected in series with the terminals of the circuit on load. In this way, it is located in the circuit being protected. It operates with or without a time delay and can be built for alternating or direct currents. The electrically isolated secondary coil allows the circuit to be tripped by using either AC or DC current supplied by the control equipment or a remote monitoring system. Tripping is instantaneous. In this way, in addition to the advantages of hydraulic-magnetic circuit breakers described above, the Ducon system provides the capability to trip a circuit of up to 100 A by remote control using a separate low voltage. It is important to note that the electrical insulation of the two coils (electrically insulated up to 2500 V) removes any risk either to human life or to the equipment, as no side effects can be transmitted from the load carrying circuit to the control circuit. This is particularly important when, for reasons of safety, voltages under 48 V are used to control different loads. Furthermore the value of the current coil of the circuit breaker may be stepped from 0.02 A up to 100 A, direct or alternating current, using various time delay periods. The Ducon coil may be constructed for voltage ranging from 2.5 V to 250 V, D.C. or 50/60 Hz or 400 Hz. "High-Inrush" Circuit Breakers The manufacture of circuit breakers for high trip currents, known as "High-Inrush" devices, is based on the same hydraulic-magnetic principle described above, and has been used successfully by our company for many years. This special operation ensures not only complete safety against overheating of machines, motors, transformers, etc., but also avoids spontaneous tripping on start-up or shutdown of circuits. The addition of a magnetic squench to the coil system allows this type of circuit breakers to withstand without nuisance tripping a current surge of up to 22 times the nominal value for a period of one half cycle at 50 Hz, or 25 times the nominal value for a period of one half cycle at 60 Hz, whilst maintaining normal overload and short circuit protection. Among the most frequently encountered applications are the protection of transformers, motors, power supplies of direct current to a highly capacitive or inductive load and fluorescent lamp circuits. The common feature of all these applications is a very high start-up or shutdown current. Start-up of a motor, depending on its type of power, may result in a current of 150 % to 600 % of the nominal value for a period of 0.1 to 2 seconds. Similarly, the switch-on current of a transformer is likely to exceed twenty times its nominal value for a few milliseconds. For all these applications, Eaton-Heinemann is able to offer a range of suitable time delay curves, designed to resolve every problem presented by high transient currents. How to choose the Right Circuit Breaker In order to choose the appropriate circuit breaker and time delay for each application the user needs to be aware of, or to determine the following: 1. The nominal current of the In circuit. This value allows the appropriate circuit breaker to be selected from the manufacturer's catalogue. The JS type will be taken, for example, for nominal AC currents up to 30A. The value of In also determines the circuit breaker's nominal current. 2. Maximum overcurrent Ip at switch-on or shut-down and its duration tp. 3. The maximum time tm during which the protected circuit can stand a specified load. 4. For motor protection, it is also necessary to know the locked rotor (start/stall) maximum current Ib, as well as the time period tb, during which the motor can stand this without damage. 5. In order to choose the frame size and the interrupt capacity current it is important that the maximum potential short circuit is known. With all this information, it is then possible to determine the most suitable time delay from the catalogue. In conclusion, it can be stated that Eaton-Heinemann hydraulic-magnetic circuit breakers offer reliable and efficient circuit and equipment protection and are notable for their ease of installation and connection. This allows them to be used in many different fields and applications, ranging from computer systems, telecommunication, industrial applications, robots, to marine, rail and air transportation.

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