Mov for line discharge

 Варисторы оксидно-цинковые Movs manufacturer Arresters which used TGEMOV have below advantages: --With the same referencevoltage, hold more excellent protective characteristics; --With the same protectionlevel, arrester have higher reference voltage andcontinuous operating voltage. Therefore, arrester could withstandlower electric stress and for safe operation.

metal oxide varistor basics metal oxide varistor theory

Basic Specification

type: metal oxide varistors/ Blocks

size: diameter38mm, height 29mm

coating: organic coating and glass coating

rated voltage: 4.5kV

craft: metal oxide

package type: wooden cases and carton

NDC:5kA

Description

DC U1mA voltage is 6.38+0.3kV

Reference current is 1mA

Reference voltage is 6.38kV

Residual voltage is 11.5+0.5kV

Long duration current is 400A

Nominal discharge current is 10kA

Applications:

Used for distribution arresters.

Arresters which used TGE MOV have below advantages:

--With the same referencevoltage, hold more excellent protective characteristics;

--With the same protectionlevel, arrester have higher reference voltage andcontinuous operating voltage. Therefore, arrester could withstandlower electric stress and for safe operation.

Варисторы оксидно-цинковые

metal oxide varistor basics 

metal oxide varistor theory

The basic properties of metal oxide blocks

(1) metal oxide blocks has protection feature, when the impact of the strong impact of the source does not exceed a predetermined value, the varistor voltage limit must not exceed the protected object can withstand voltage shock resistance.
(2) the impact resistance, the varistor itself should be able to withstand the impact of current, impact energy, and several attacks have appeared when the average power.
(3) There are two characteristics of life, one continuous working voltage life, namely varistor specified ambient temperature and system voltage conditions should be able to work reliably specified time. Second, the impact of life, that is able to reliably withstand the impact of a predetermined number of times.
(4) After the metal oxide varistor basics intervention system, in addition to protect a "safety valve", it will also bring some additional impact, which is the so-called "secondary effects", it should not reduce the normal performance of the system. Then the main factors to be considered, there are three, one varistor own capacity, the second is the system voltage, leakage current, the third is non-linear current through the varistor coupled source impedance effects on other circuits.

The note of ZnO Resistors

Temperature fuse ZnO Resistors should have good thermal coupling when varistors fail (high-impedance short circuit), the heat it generates the temperature fuse fuse, varistor and separation circuit failure to ensure that the equipment Safety. When a high frequency temporary over-voltage applied to the varistor , the varistor may cause momentary short-circuit breakdown (low impedance short circuit), the temperature fuse fuse had a chance, but also may cause fire. To avoid this phenomenon may be further connected in series to each varistor fuse a high impact frequency (single frequency fuse employment at the time of aging failure may not fuse)

The role of Zinc blocks

Zinc blocks are widely used in electronic circuits, because of the protection circuit to damage the instantaneous voltage power supply system of the mutation may be. When the high pressure comes, the varistor resistance decreases and current shunt to be safeguarded against excessive transient voltage damage or interference. Thereby protecting sensitive electronic components.
However, I do not think the role of the varistor too, varistors are not able to provide a complete voltage protection varistors can withstand the energy or power is limited, do not provide sustained overvoltage protection . Sustained overvoltage will destroy the protection device (varistor), and damage to equipment, and there may be a fire hazard.Zinc Oxide block not provide protection part are: the inrush current at power-on, over-current short circuit, voltage dips, etc., these cases require other forms of protection. In addition, there are a few protection device detects the power supply voltage, such as the power supply will be cut off when the relay exceeds dangerous limits.

Metal Oxide Varistors (MOV)

While an Metal Oxide Varistors (MOV) is designed to conduct significant power for very short durations (about 8 to 20 microseconds), such as caused by lightning strikes, it typically does not have the capacity to conduct sustained energy. Under normal utility voltage conditions, this is not a problem. However, certain types of faults on the utility power grid can result in sustained over-voltage conditions. Examples include a loss of a neutral conductor or shorted lines on the high voltage system. Application of sustained over-voltage to a mov metal oxide varistor can cause high dissipation, potentially resulting in the MOV device catching fire. The National Fire Protection Association (NFPA) has documented many cases of catastrophic fires that have been caused by MOV devices in surge suppressors, and has issued bulletins on the issue.

A 130 volt, 150 J MOV that has undergone catastrophic failure, apparently as a result of a lightning strike, showing evidence of heat and smoke. The 3 amp fast-blow fuse immediately in front of the varistor blew during the same event.
A series connected thermal fuse is one solution to catastrophic MOV failure. Varistors with internal thermal protection are also available.
There are several issues to be noted regarding behavior of transient voltage surge suppressors (TVSS) incorporating MOVs under over-voltage conditions. Depending on the level of conducted current, dissipated heat may be insufficient to cause failure, but may degrade the MOV device and reduce its life expectancy. If excessive current is conducted by a MOV, it may fail catastrophically, keeping the load connected, but now without any surge protection. A user may have no indication when the surge suppressor has failed. Under the right conditions of over-voltage and line impedance, it may be possible to cause the MOV to burst into flames,[5] the root cause of many fires[6] and the main reason for NFPA’s concern resulting in UL1449 in 1986 and subsequent revisions in 1998 and 2009. Properly designed TVSS devices must not fail catastrophically, resulting in the opening of a thermal fuse or something equivalent that only disconnects MOV devices.

The Specifications of metal oxide varistor application

MOVs are specified according to the voltage range that they can tolerate without damage. Other important parameters are the metal oxide varistor application energy rating in joules, operating voltage, response time, maximum current, and breakdown (clamping) voltage. Energy rating is often defined using standardized transients such as 8/20 microseconds or 10/1000 microseconds, where 8 microseconds is the transient's front time and 20 microseconds is the time to half value.
Response time
The response time of the varistors is not standardized. The sub-nanosecond MOV response claim is based on the material's intrinsic response time, but will be slowed down by other factors such as the inductance of component leads and the mounting method. That response time is also qualified as insignificant when compared to a transient having an 8 μs rise-time, thereby allowing ample time for the device to slowly turn-on. When subjected to a very fast, <1 ns rise-time transient, response times for the MOV are in the 40–60 ns range.
Capacitance

Typical capacitance for consumer-sized (7–20 mm diameter) varistors are in the range of 100–1,000 pF. Smaller, lower-capacitance varistors are available with capacitance of ~1 pF for microelectronic protection, such as in cellular phones. These low-capacitance varistors are, however, unable to withstand large surge currents simply due to their compact PCB-mount size.

The Applications of High voltage varistor

To protect telecommunication lines, transient suppression devices such as 3 mil carbon blocks (IEEE C62.32), ultra-low capacitance varistor metal oxide , and avalanche diodes are used. For higher frequencies, such as radio communication equipment, a gas discharge tube (GDT) may be utilized.[citation needed] A typical surge protector power strip is built using xatge MOV . Low-cost versions may use only one varistor, from the hot (live, active) to the neutral conductor. A better protector contains at least three varistors; one across each of the three pairs of conductors. In the United States, a power strip protector should have an Underwriters Laboratories (UL) 1449 3rd edition approval so that catastrophic MOV failure does not create a fire hazard.