Hager General Catalogue

4.28

General Catalogue • Protection Devices

Surge Protection

Risks of Electrical Surges

Surge protective devices (SPD)

assist in the protection of

valuable electrical and electronic

equipment against transients,

originating from lightning and

also from switching sources.

These transients can cause

damage ranging from the

premature ageing of equipment,

logic failures and down time,

to the complete destruction

of equipment within the entire

electrical installation. Products

such as LCD screens, data

servers and industrial equipment

such PLC’s are critical to

business activity. Protecting

this equipment may now be a

necessity.

The Hager SPD range of

solutions may offer protection

to prevent damage to this

sensitive equipment by diverting

the damaging transient over-

voltages. In the majority of cases

this will eliminate equipment

failures and reduce downtime.

The choice of a surge protective

device depends upon:

• The exposure of the building

to lightning transients

• The sensitivity and value of

the equipment that requires

protection (it is recommended

that the contractor should

discuss the installations

requirements with the customer)

• The location and therefore

the exposure level of the

installation

• The equipment used within

the installation and whether this

equipment could generate

switching transients

BS 7671 and the AQ criteria

method

Lightning discharges could

contain currents of 200,000A

which if struck at or near

power transmission lines

would generate a significant

voltage transient. This voltage

transient could cause significant

damage to both domestic

and commercial electronic

equipment.

The UK regional map illustrates

the likely lightning activity

caused by the number of

thunderstorm days across the

country.

Protection against over-voltages

is the subject of section 443 of

BS 7671. Here the AQ criteria

method is introduced which is

based on the likelihood of the

equipment being subjected

to over-voltages caused by

lightning strikes, taking account

of the probable number of

lightning strikes per year.

For electrical installations

in the UK, the map shows

that the probable number of

thunderstorm days per year in

any given location is less than

25, and therefore condition AQ1

applies.

Where this is the case and for

installations being supplied

by overhead lines, Regulation

443.2.2 indicates that provided

the impulse withstand voltage of

the equipment is not less than

the values given in Table 44.3

(see Table 1 for installations

rated at 230 V to Earth), no

additional protection by a SPD is

required. However, where higher

levels of equipment reliability

or higher risks (e.g. fire) are

expected, additional protection

by an SPD against over-voltage

may be required.

Similarly, for an installation

having overhead lines, no

additional protection against

overvoltages is required if the

equipment meets the minimum

voltage withstand values in table

44.3.

There are some words of caution

in the notes to this section where

it is recognised that transient

over-voltages transmitted by

the supply distribution system

are not significantly attenuated.

So an induced voltage some

distance away could easily

manifest itself at the electrical

installation and cause potential

harm to the equipment within.

It is also worth considering that

the AQ data is for thunderstorm

days NOT lightning strikes. One

storm will usually contain many

lightning flashes which could

lead to an over-voltage on the

installation causing damage to

equipment.

Cascading

Cascading is the term used

to describe the method of

combining several levels of surge

protection devices into the one

installation.

This takes advantage of the

best features of each device

to improve the protection

level for the equipment. Hager

recommends using a high surge

current capacity device to

divert the bulk of the transient

over-voltage at the origin of the

installation.

In the case of a Class 1 & 2

device this would be either the

spark gap arrester or a high

current capacity MOV. Should

finer protection be required, the

next step is to install a Class 3

device SP202N near the terminal

equipment.

Cascading increases the current

diverting capacity of the SPD

system whilst maintaining a

low voltage (Up) to ensure the

best protection for valuable

equipment.

Selecting SPD of the same

manufacturer or make will ensure

correct co-ordination between

devices.

SPD Quick Selection Guide

The following is a quick selection

guide which may assist

in choosing whether SPD’s are

required and the correct

type of device

• Does the installation contain

a lightning protection system?

• Is the installation adjacent to

any tall structure, tall trees or

near a hill top in a lightning prone

area?

• Does the installation contain

equipment where higher

reliability from overvoltages is

required

If the answer is YES in the above

to the first two questions, it is

recommended to install a Type

1+2 device. This will provide

protection against surges caused

by direct lightning strikes and

provide protection against

transient over-voltages caused

by indirect lightning strikes or by

switching events.

If the answer is YES to the third

question then it is recommended

to install Type 2 devices to

provide protection against

transient over-voltages caused

by indirect lightning strikes or by

switching events.

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