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Understanding SPD Terminologies

Understanding SPD Terminologies
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Technical Articles

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Surge Protection Devices are defined by various electrical specifications, Understanding these specifications will help the user to select the right surge protection device specific to their installation/application.

Un - Nominal Operating Voltage

The nominal voltage stated for a SPD corresponds to the system voltage of the typical SPD installation site, e.g. 230/400V AC

Uc - Maximum Operating Voltage

  • Maximum r.m.s value of the voltage which is allowed to be continuously applied to the terminals of SPD in the event of surge
  • Once voltage across SPD terminals will go above this value, SPD will change its state from open circuit to closed circuit mode.
  • Uc value must always be equal to or higher than Un rating.
Maximum Operating Voltage

Imax- Maximum Discharge Current

  • Imax is the term associated with Type-2 SPD. It is the Peak current of 8/20μs wave shape a surge protector can withstand without destruction.
  • IEC 61643 - I standard (Standard for low voltage surge protective devices) has defined below 8/20μs current waveform to simulate switching surges/surges due to indirect lightning strikes.
IEC 61643

Iimp - Maximum Lightning Surge Current

  • Iimp is the term associated with Type-1 SPD. It is the Peak current of 10/350μs waveform a surge protector can withstand without destruction.
  • IEC 61643-I standard (Standard for low voltage surge protective devices) has defined below 10/350μs current waveform to simulate direct lightning strikes.
Maximum Lightning Surge Current

Note: Type 1+2 SPD gives combined protection of Type 1 and Type 2 SPD (Protection against Direct/Indirect lightning strikes) . Hence, for Type 1+2 SPD, Iimp and Imax both the terms get associated

UP –Voltage Level of Protection

  • It is the Maximum voltage that can occur across the connection terminals of a SPD.
  • In the event of surge, when surge current and surge voltage are within the performance parameters of the SPD, the voltage is safely limited to a maximum of this value across the terminals of the SPD.
  • Up value must be lower than load’s Uimp (Load’s impulse withstand voltage)


Also Read: What is SPD? Its Need & How a Surge Protection Device Works

In - Nominal Discharge Current

  • The nominal discharge current (In) is the peak current in 8/20μs wave shape a surge protector can withstand repeatedly (15 surges) without destruction.
  • In value is always less than Imax value of SPD
Maximum Lightning Surge Current
Type 1Type 1+2Type 2Type 3
Nominal Operating Voltage (Un) ✓ ✓ ✓ ✓
Maximum operating voltage (Uc) ✓ ✓ ✓ ✓
Voltage Level of protection (Up) ✓ ✓ ✓ ✓
Temporary Overvoltage (UT) ✓ ✓ ✓ ✓
Max. Lightning Surge Current (Iimp) ✓ ✓ ✗ ✗
Max. discharge current (Imax) ✗ ✓ ✓ ✓
Nominal discharge current (In) ✓ ✓ ✓ ✓
Short-circuit withstand capacity (ISCCR) ✓ ✓ ✓ ✓

UT-Temporary overvoltage

The temporary overvoltage UT (TOV) is the maximum r.m.s. value, the surge Protector can withstand for several Seconds, without failure. In SPD, UT is typically equal to or higher than UC.

ISCCR - Short Circuit withstand capacity

ISCCR indicates the maximum prospective short-circuit current of the electrical network in which SPD is getting connected in conjunction with the upstream overcurrent protective device. Prospective short-circuit current is the highest electric current which can exists in a particular electrical system under short-circuit condition.

ISCCR of SPD needs to be higher than the short circuit value of the network of at SPD’s installation point.

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