Handbook of Electrical Installation Practice - BeKnowledge

242 Handbook of Electrical Installation Practice
Table 9.17 High impulse current tests.
Derived from ITU IX K17 and BS 6651: 1999.
Transients can occur between any combination of conductors. Manufacturers
should therefore make quite clear which modes of transient propagation let-through
performances relate to, e.g.:
• Live/phase(s) to earth, live/phase(s) to neutral and neutral to earth for mains
power supplies
• Line to line and line(s) to earth for data lines.
The maximum transient current the protector can withstand should be clearly
stated, e.g., 20000A, 8/20 ms.Again, this is a test value for the whole protector and not
a theoretical value. The maximum transient current of the protector also takes
account of its energy handling capability. Energy ratings, as a sole indicator of protective
performance, are misleading, as the energy deposited in a protector by a transient
current source depends on the suppression level (i.e.let-through voltages).Thus,
lower energy ratings do not necessarily mean that the protector is less likely to
survive.
The international telecommunication standard also calls for high impulse current
testing of data line protectors for telephone lines. This test (within ITU IX K12) is
intended to test the capability of gas discharge tube(s) present in most data line protectors.
Test levels are shown in Table 9.17.
Any factor that may interfere with the system during normal operation should
be quoted. These may include the following:
• Nominal operating voltage
• Maximum operating voltage
• Leakage current
• Nominal current rating
• Maximum continuous current rating
• In-line impedance (or resistance)
• Shunt capacitance
• Bandwidth
• Votage standing wave ratio (VSWR) or reflection coefficient.
Table 9.18 summarises potential causes of incompatibility or system impairment for
different types of power and data line protectors. Protector manufacturers should