Why install a SCB (Surge Circuit Breaker) in front-end of SPD (Surge Protective Device)

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Why install a SCB (Surge Circuit Breaker) in front-end of SPD (Surge Protective Device)


What is SCB?

SCB – Surge circuit breaker or SPD backup protector

Why SCB?

SCB successfully solved the worldwide problem of surge protective device failure ignition trip.

Product Usage

  1. Selective disconnection of the passed power frequency current and lightning current can effectively protect the SPD from short-circuiting and short-circuiting of SPD due to abnormal transient over-voltage, resulting in serious fire accidents.
  1. The selective division of the passed power frequency current and lightning current can effectively protect the SPD from causing the SPD starting voltage to drop below the power supply voltage, and the power frequency leakage current increases, causing a serious fire accident.
  1. When the SPD has lightning current, the external disconnector will not be accidentally tripped, so that the lightning protection of the electrical equipment is always in an effective state.

Scope of application

The SCB dedicated backup protector provides professional backup protection for the SPD (Lightning Protection Device) that protects the power supply of the first, second and third levels. Applicable to places where SPD lightning protection equipment has been installed, such as power equipment for industrial and civil construction, electrical, communications, road transportation, petrochemical and other industries.

Working principle

SCB, the exclusive external disconnector of SPD, is a kind of equipment which is developed according to the article 430.3 in the IEC61643-4-43: adopt suitable over – current protection devices before dangers caused by circuit. It mainly solve problems that when follow currents or leakage currents take place in SPD, SCB can trip quickly ,while lightning currents pass, SCB does not trip, SCB ensures that SPD does not cause a fire and the lighting protection of equipment lasts long, solving the problems that there is protection blind in current widely used fuses and breakers which are used as external disconnectors. SCB is the ideal matching devices of voltage switching type SPD, voltage limiting type SPD used low-voltage power supply system.

All over the world are taking part in solving the problem:

When the ignition of SPD takes place, external breakers do not disconnect, And when surge flows through SPD, external breakers disconnect mistakenly.

German conducted the experiment of lightning current impact on fuses and breakers in 1997. Green area means connection, orange area means uncertainty, and red area means disconnection.

experiment of lightning current impact on fuses and breakers

IEC drafted and revised the standard of SPD. Sub-committee 37A set up Task Force 12 in Austria-Vienna meeting. solving the matching problem between breakers and SPD.

Many countries around the word launched the study the degeneration problem of MOVs (metal oxide varistors) SPD.

deterioration mechanism pic1

  1. When the degeneration of SPD takes place, the display of electrical parameters is that Uc value reduces.
  2. When Uc value reduces to the voltage of power, leakage current will increase sharply.
  3. When power appears abnormal temporary over-voltage, it will cause SPD to start.
  4. When the normal current of more than 5A flows through SPD, the speed of ignition is faster than heat transfer.

When current more than 5A passes through SPD, it may catch fire immediately, so the SPD needs a switch protector which releases quickly when the current of more than 5A passes through to avoid fire!

5A-300A passes through SPD

It demands that when lightning current flows through, it does not trip, which keeps the effectiveness in the work.

When the degeneration of SPD takes place or before leakage current caused by abnormal power reaches 5A, it can trip quickly.

SCB action curves of safety tripper

What can SCB solve the problem

The mismatching between lightning protection devices and fuse or breaker?

The traditional method is to link a fuse or breaker in series in front of lightning protection devices, there would be four mismatching aspects if doing so.

  1. When lightning protective devices degrade or there is overvoltage occurring in the distribution circuit, lightning protective devices will become short-circuited to grounding and Fuses or breakers can’t disconnect quickly
  2. When lightning occurs, fuses or breakers can’t stand temporary energy of lightning current, for they were used as components of power distribution in the early design. So it is easy to cause them to trip or explode, making lightning protective ineffective.
  3.  When lightning current goes through breakers, the Up value is very high and lightning protective devices can’t protect equipment well.
  4.  Fuses or breakers can’t disconnect in the power be installed line of a transformer. When a short circuit takes place, they can’t break quickly.

SCB can solve four problems at the same time

SCB, linked in series in front of SPD can solve four problems at the same time.

  1. When lightning protective devices degrade or there is overvoltage occurring in the distribution circuit, SCB can disconnect quickly to avoid lightning protective devices from firing. The breaking current is less than 3 A.
  2. When lightning current goes through, the SCB linked in series in front of SPD can keep no tripping and no damage under the lightning current of 100kA, keeping the SPD in working order.
  3. When lightning current goes through SCB, the Up value is very low, be equal to copper with the same length.
  4. The breaking capacity of SCB exceeds plastic breakers, up to 100kA.

There is a big difference in time and amplitude between power frequency current and surge. SCB makes good use of this two parameters to control electromagnets, achieving the function of tripping.

  1. When alternating current flows through, the electromagnet can trip selectively to cut off alternating current.
  2. Because the speed of surge is too fast, surge ends before the electromagnet takes action. So electromagnet is in the stable state and SCB does not trip.

The residual voltage of SCBs, MCBs and Fuses under impulse current

The residual voltage of SCBs, MCBs and Fuses

Typical applications

Typical applications

SCB connection with SPD basic circuit diagram

SCB series connection with SPD Basic circuit diagram

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