Handbook of Electrical Installation Practice - BeKnowledge

typically consists of semiconductor protection installed at board level and gas discharge
tubes installed on the main distribution frame. Certainly this is hardly ever
done in the UK nor, we think, in most other parts of the world. If lines are correctly
protected this tends to be true of the incoming (PTO) telephone lines only and not
extension lines. These often travel from building to building and are therefore very
much at risk from lightning.
The use of gas discharge tubes alone does not provide adequate protection. The
PBX should be properly protected by devices with a suitably low let-through
voltage.The PBX is an excellent place to install protection because all incoming and
outgoing lines meet at this point.
Choosing and specifying transient overvoltage protectors
In selecting a transient overvoltage protector it is important that the device selected
survives, has a suitable transient control level, and is compatible with the system
it is protecting. It is vital that the protector chosen is capable of surviving the
worst case transients expected at its intended installation point. Also, as lightning is
a multiple pulse event, the protector should not fail after exposure to the first
transient.
The protector should be able to control transients to level below the susceptibility
and vulnerability of the equipment to be protected; for example, if a computer’s
operation is unhindered by transients of up to 700V, then the let-through voltage
or transient control level of the protector should be less than 700V. Allowing a
suitable safety margin, the worst case let-through voltage of the protector should
be 600V, or less. (Note, the connecting leads of a correctly installed protector will
cause an increase in the let-through voltage.) Also the protector should not impair
or interfere with the protected system’s normal operation – something which
communication systems and intrinsically safe circuits are particularly susceptible to.
Location categories
The protector’s ability to survive and to achieve a suitable ‘let-through’ voltage
clearly depends on the size of transient to which it will be subjected. This in turn
may depend on the protector’s location. The American Standard IEEE C62.41 and
subsequently BS 6651 outline three location categories.
As a mains-borne transient travels through a building, the amount of current it
can source grows smaller (owing to the impedance of mains cables and current division).
This is based on the assumption that a typical mains transient caused by lightning
has a 1.2/50ms waveshape.
Location Category C is defined as either:
Lightning Protection 239
• Outside the building, or
• The supply side of the main, incoming, l.v. distribution board, i.e. the board bringing
the power supply into the building from the electricity supply authority, l.v.
transformer or from another building, or
• The load side of distribution boards providing an outgoing power supply to other
buildings or to on-site equipment.