Handbook of Electrical Installation Practice - BeKnowledge

240 Handbook of Electrical Installation Practice
Location Category B is defined as either:
• On the power distribution system, downstream of Category C and upstream of
Category A, or
• Within apparatus which is not fed from a wall socket, or
• Sub-distribution boards located within a 20m cable run of Category C, or
• Plug-in equipment located within a 20m cable run of Category C.
Location Category A is defined as either:
• Sub-distribution boards located more than a 20m cable run away from Category
C, or
• Plug-in equipment located more than a 20m cable run away from Category
C.
Transient overvoltages on data lines are not significantly attenuated by the cable
and so are always rated for Location Category C. Regardless of where they are
installed in the building, the worst case will be similar. This is based on a 10/700 ms
waveform transient.
Exposure levels
Transient overvoltage protectors are designed to protect against the probable worst
case transient overvoltage. In high transient exposure level area, very large transients
(perhaps only occurring once in every few thousand events) can be anticipated
over much shorter time-scales than in a low transient exposure level area.
Thus, the probable worst case transient will be much smaller in a low transient exposure
level area, than in a high exposure area.
The transient exposure level can be derived from the risk assessment – see
earlier. However, if a risk assessment has not been done, it is probably wise to
assume a risk, R = 0.6. This can be combined with the consequential loss rating
(in Table 9.13) in order to derive the protector exposure level from Table 9.14.
Probable worst case mains transients for high, medium and low exposure levels
are tabulated in Table 9.15 for all location categories. All data line protectors fall
within Category C and probable worst case data line transients are tabulated in
Table 9.16.
Protector performance
These location categories and their probable worst case transients, provide us
with a yardstick with which to evaluate protectors. Subjecting the protector to an
appropriate transient test enables us to determine whether it will survive. The
protectors let-through voltage for this test tells us its transient control level. Letthrough
voltage is a measure of the amount of the transient overvoltage which gets
past, or is let-through, the protector. Thus, if a protector is required for a mains