ENG
EST
18.09.2015

Voltage Relay

Electric power quality. CBEMA curves
Currently, enormous amount of the most various household appliances are in operation, which year after year are equipped with more difficult electronics. The stable operation of all electrical equipment depends completely on the electric power quality. Most problems of electronic equipment in the home and in industries are related to the sharp failures and peak surges of network voltage. CBEMA curves (Computer and Business Equipment Manufacturers Association) have been created by trial and error to reduce the number of electronic equipment failures due to unstable voltage, which describe the ability of equipment to maintain voltage deviations from nominal value. The curves show dependence of the deflection value of supply voltage on a time lag of this influence and determine an area within the limits of which the electronic equipment must operate continuously and without failures.

Red lines in figure show maximum and minimum voltages, which do not lead to failures in equipment operation in relation to time. Ideally, such curves should describe the actual indexes of the power network and producers of electronic equipment should adapt to such actual data. The problem resides in the fact, that if equipment of most producers really meets these requirements, then the same cannot be said of the actual indexes of the electric power quality in the power supply systems.
There seemed no escaping the conclusion that a protection device is required in order to provide the electronic equipment safety at the network voltage disturbances. The protection device must have the same characteristic (with a small margin to eliminate faulty actuations). It means, when power network voltage overruns the curve green area, the protection device should disconnect electronic equipment from the network.
Such device is a voltage relay – a protection device capable in a fraction of a second to de-energize consumers when the supply voltage overruns permissible limits.
In the home, such devices appeared in more simplified and completed form: this is a device ready to operate and designed to protect household appliances against dangerous levels of the power network voltage.
A voltage relay represents an electronic device of a voltage monitor and a load disconnector power circuit assembled in one case.
The voltage monitor can be manufactured based on a comparator (a comparing device) or microprocessor.
The circuits wired up on the base of a comparator are cheaper and manufactured with the fixed operation thresholds. An operation threshold is a level of low or high input voltage after reaching of which a relay will disconnect the power supply of the output circuit. The fixed thresholds are preset, as a rule, by a manufacturer in accordance with a working standard (220 V ± 10 %).
The microprocessor-based voltage relays differ by smooth regulation of high and low operation threshold. The operation thresholds are not fixed in the relay of such type, i.e. they can be changed by a user as may seem expedient to him (for example, for the equipment, which requires more exact level of input voltage ±5 %).
The main parameter of a voltage relay is a response speed. The response time of a relay must provide the connected equipment safety.
The power circuit of a device must provide the continuous reliable power supply of a consumer without heating of power contacts.
Practically every relay has a time delay function of load connection to avoid a transmission of surges and distortions of sinusoidal voltage in the load, which arise during network connections. As is the case with operation thresholds, the time of delay can be fixed by a manufacturer, or the regulation function can be built up.
More expensive models of voltage relays have the opportunity to regulate the time of delay. Such relays are suitable for operation jointly with refrigerating and climatic equipment.
It depends on the real danger of the compressor failure even with the brief voltage miss in the power supply network. Thus, for example, the cooling agent during a refrigerator or air-conditioner operation is under high pressure created by the compressor. The sudden disconnection of the power network will result in the stop of an electric motor and in creation of the pressure difference in different parts of the system. For some time the pressure will be high on the compressor output. In this connection after quick power supply renewal the electric motor will be in the blocked state, which can result in the equipment damage.
Contrary to the automatic voltage regulator a relay does not regulate the power supply voltage, but only disconnects a protected area at the increase or decrease of voltage and automatically connects it after voltage stabilization in the network. Therefore, it is very effective during emergencies arising as a result of wire breaks, overload, skew of phases etc.
Depending on a connection type, a voltage relays are subdivided into single-phase and three-phase.
The single-phase voltage relays are designed to protect the single-phase load against impermissible voltage oscillations in the power supply network. Independent switching devices can be used, as well as to control other switching instruments, for example, magnetic starters.
The three-phase voltage relays are used to protect three-phase consumers against impermissible voltage oscillations in the power supply network, break and skew of phases, agglutination and disturbance of phase sequence. They can be used jointly with devices, where the presence and quality of phases must be controlled continuously.

Reasons to install voltage-check relays
According to a standard, the voltage deviation 10 % from a norm 220 V is allowed, i. е. if the voltage is from 198 to 242 V in the power supply network – then it is a norm. Low voltage is frequently related to the wear of power lines and increased load on them. High voltage is rarer, but it is more dangerous. Sometimes the sudden voltage disconnection is accompanied with pulse interferences, when voltage and current surges occur. Rarer, but however sometimes there are cases, when 380 V voltage instead of 220 V is supplied in the house electric networks, as a result there is a strong possibility of failure of home electrical appliances. If someone can think about purchasing or not a protection device against low voltage, then a protection device against high voltage must be installed by all means..
High voltage occurs also because of emergency condition of wiring in apartment houses, when due to the breakage of the common neutral wire, nearby phases appear under dangerous 380 V voltage. The single-phase voltage is supplied to apartments from the three-phase network (neutral – phase). On the neutral wire breakage, the voltage will depend on load to nearby phases. As the load is different, the voltage on household electric appliances will be different – up to 380 V. The considerable exceeding of the normal input voltage level leads to failure of appliances and their possible inflammation. Many people leave their apartments without a moment’s thought, keeping TV sets (in standby mode), musical centers, washers and dish-washing machines, refrigerators, computers and routers connected to the mains. Installation of a voltage-check relay can be a possible solution of the problem.

Voltage-check relay selection
A high-capacity relays are mounted as a rule on a DIN rail, they must be mounted by an electrician, sometimes it is not convenient. It should be remembered that such a voltage relay does not have the built-in protection against high currents. An automatic circuit breaker should be mounted ahead of the voltage relay on the DIN rail. The automatic circuit breaker is selected with the rating 20-30 % less than the relay rating.
When point consumption sources have to be protected, then it is more appropriate to purchase a voltage relay with plugging in a socket. They do not require mounting and are easy-to-use

Single-phase voltage relay with a possibility to regulate the time of delay
and operation thresholds.

Such voltage relay is plugged directly in a socket and used to protect separate consumers or their groups. The relay is controlled by a microcontroller, which analyses the supplied voltage and represents its virtual value on a digital display. The load is cut off by an electromagnetic relay. The buttons (see the User’s Manual) are used to preset tolerable limits of the input voltage and time of delay.

Single-phase voltage relay with fixed operation thresholds and time of delay

Such voltage relay is also plugged directly in a socket and used to protect of separate consumers or their groups. Its circuit analyses the supplied voltage. The load is cut off by an electromagnetic relay. Limits and time of delay have the fixed values are not regulated.

The voltage relay is one of those devices, which is desirable in every house and in general anywhere, where mains-operated electrical appliances or devices are available. They are the most effective in case of emergency due to broken wires, overload, distortion of phases etc.

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