Recommendation K.22



OVERVOLTAGE RESISTIBILITY OF EQUIPMENT 

CONNECTED TO AN ISDN T/S BUS





1.	General



	This Recommendation seeks to establish fundamental testing methods and criteria for the resistibility 
of telecommunication equipment connected to an internal ISDN T/S bus.



	Recommendation K.21 should be followed when assessing the resistibility of equipment to be con-
nected directly to a telecommunication network.



2.	Scope



	The Recommendation relates to any terminal equipment which is intended to be connected to the 4-
wire T/S bus of an ISDN installation. It presumes that suitable isolation is provided between the telecom-
munication network and the T/S bus at the network termination. It is also assumed that the S-bus has no 
connection to earth e.g. no earth-connected voltage-limiting devices with non- linear characteristics can 
be used. In cases where these assumptions cannot be made, Recommendation K.21 should be fol-
lowed.



3.	Overvoltage and overcurrent conditions



	Aspects of overvoltage or overcurrent covered by this Recommendation are:



	-	surges due to lightning strokes on telecommunication lines or to the building housing the 
equipment;



	-	electrostatic discharges generated by users touching the equipment or adjacent plant;



	-	lightning transient surges on mains - voltage power supplies to the equipment.



4.	Equipment boundary



	Variations in equipment make it necessary for each unit to be seen as a "black-box" having three or 
more terminals, A, B ... etc. Some protective devices may have already been provided within the equip-
ment, e.g., distributed on cards, or connected to internal terminals. For the purpose of these tests manu-
facturers are expected to define the boundaries of the "black-box" and any protective device which is 
thereby included must be considered an immutable part of the equipment.



5.	Test conditions



	The following general conditions apply:



5.1	All tests are type tests.



5.2	The input terminals at which tests on the equipment are to be applied should be identified by the 
manufacturer and labelled A, B ....



5.3	The equipment may be enclosed in a foil shroud which is connected to the earth terminal (if it is pro-
vided), when tests are made under __ 7 and 9.

5.4	The equipment should be tested in each operating mode of significant duration.



5.5	The equipment should pass tests under __ 7 and 9 throughout the ranges of temperature and humid-
ity of its intended use.



5.6	In all cases where a maximum voltage is specified, tests shall also be made at lower voltages if it is 
necessary to confirm that the equipment will resist any voltage up to the maximum specified.



6.	Test compliance



	Equipment shall withstand all tests without damage or other disturbance, e.g., corruption of software, 
misoperation or fault-protection facilities, and shall operate properly within specified limits after the tests. 
It is not required to operate correctly while the test condition is present. If specifically permitted by an 
administration, the tests may cause the operation of fuses or other devices which have to be replaced or 
reset before normal operation is restored.



7.	Surge tests



7.1	Test circuits



	Three alternative test circuits may be used:



	-	a surge generator of 1.2/50 µs open-circuit voltage waveshape and 8/20 µs short-circuit cur-
rent waveshape;



	-	a surge generator providing a 2/10 µs open-circuit voltage waveshape and short-circuit current 
waveshape;



	-	a surge generator of 1.2/50 µs open-circuit voltage waveshape and corresponding short-circuit 
current waveshape. Figure 1 illustrates a typical test circuit.



	The short-circuit current provided by the surge generator shall be approximately 100 amps.