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Surge Arrestors

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Voltage surges have different charactersitics

Types of voltage surges

Atmospheric (lightning)

Switching

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A voltage surge is characterised by its rise time and duration

Types of voltage surges

Transient overvoltages: atmospheric several MHz; 1 to 100 s)

switching (circuit breaker, motors, fuses,

etc.) (100KHz to 1MHz; 0.05 to 10 ms)

Temporary overvoltages: 50Hz with a duration of 0.03 to 1 second

temporary voltage

surge

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Electrical equipment is tested to standard impulse voltages to

determine withstand level

Standardised impulse waves

Standardised voltage wave

1.2 / 50s

Standardised current wave

8 / 20s

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All electrical equipment has an impulse withstand voltage

Impulse withstand voltages

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Surge protection devices

Serial protection deviceThis is connected in series with the load to

be protected.

Filters, transformers, wave absorbers,

network conditioners and static UPS.

This type can eliminate network

(temporary) or transient voltage surges(50-60-400Hz to 200kHz).

Parallel protection deviceThis is connected in parallel with the load

to be protected.

Surge arrestor/diverter most often used.

Carbon spark gaps, gas discharge tubes,

zener diodes, MOVs.

This type eliminates high frequencytransient voltage surges such as lightining

impulses (MHz).

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The voltage/current characteristic of the ideal surge protection device

Surge arrestors

Leakage current 0

Residual voltage low

Energy dissipation high

Leakage current low

Residual voltage low

Energy dissipation high

The voltage/current characteristic of the metal oxide varistor

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Electrical characteristics of low voltage MOV surge arrestor

MOV surge arrestor

Uc: permanent operating full withstand voltage

(MOV rating)

Ic: leakage current typically < 1 mA

Up: protection level (residual voltage):1 kV - 2.5kV

In: rated discharge current. This is the peak value

of the 8 / 20 s wave current that can flow through

the surge arrester without any damage (20 shocks)

Imax: this is the peak value of the 8 / 20 s wave

current that a surge arrester can withstand

(1shock)

Note:

Imax or In kA not to be confused with rms

kA fault rating

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Electrical characteristics of low voltage MOV surge arrestor

MOV surge arrestor

Auxiliary contact for remote

indication

Thermal disconnector

indicator flag

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Operating principle

MOV surge arrestor (diverter)

R high

R low

()

(U)flash U

Vres

During normal operating conditions, the surge

arrestor has a very high resistance (effectively

an open circuit).

If a surge, greater than the flashover (trigger)

voltage of the surge arrestor, appears on the

network, the surge arrestors resistance drops

to a very low value (effectively a short circuit)

The surge arrestor will conduct the discharge

current to earth whilst maintain the voltage

(residual) at or below the protection level. Once the discharge energy has dissipated, and

the network voltage is restored, the resistance

of the surge arrestor rises to its initial high

value.

The type of surge arrestor must match the type

of protection required, depending on the

magnitude and frequency of the surge, and theequipment/ installation to be protected.

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Voltage surges also occur in two different modes

Types of voltage surges

Common modeCommon mode voltage surges occur between

the live conductors and earth:phase to earth

or neutral to earth

Dangerous for equipment which have an

earthed frame: dielectric breakdown.

Differential modeDifferential mode voltage surges circulate between

phase to phase or phase to neutral conductors

Dangerous to sensitive equipment:

damage to electronic components.

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The surge arrestor is made up a protection element (MOV) and a

thermal disconnector which deteriorates with energy dissipation and

eventually ruptures.

MOV surge arrestor

Common modeprotection

Common / differentialmode protection

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load

surge

arrestor

MEN linkMain neutral link

Main earth barL1L2L3N

L1L2L3N

AS4070 recommends 6mm2 cables connections (as straight as possible)

Installation as close to origin as possible

Connected between supply side or load side of main switch and:

- Neutral bar if installed upstream of MEN link

- Earth bar, if installed downstream of MEN link

Keep connecting leads as short as possible. Total length of active and earth should not exceed 1m. (50cm

rule)

Upstream protection/isolation may be provided by MCBs or fuses (follow manufacturers recommendations.

May be connected to load side of RCD, if RCD is time delayed.

Installation guidelines

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Installation guidelines

1P and 3P devices connected

upstream of MEN link (Neutral

and Earth conductors are

combined).

1P+N and 3P+N devices

connected downstream of

MEN link (Neutral and Earth

conductors are separate).