IEC 60255-5:2000 pdf free download – Electrical relays
4.2.2.1overvoltage categories
The applicable overvoltage category shall be determined on the basis of the following criteria.ategory l Category l applies for measuring relays and protection equipment where special measuresare faken to limit transient voltages to appropriate values，e.g. well-protected electroniccircuits.
Category u Category ll applies where:
a) the auxiliary circuits(power supply circuits) of the relay are connected to a voltage supply used only for the power supply of static relays. If the leads are short and in the absence ofswitching of other circuits connected to the supply，the levels of transient voltage on thesupply leads will be lower than specified in overvoltage category lll;
b) the input energising circuits of the relays are not connected directly to voltage or currenttransformers and good screening and earthing are employed on the connection leads;
c) the output circuits are connected to a load by short lead lengths. Category m
Category lll applies to most practical cases of the application of measuring relays andprotection equipment and shall, in particular, be used where:
a) the auxiliary energising circuits (power supply circuits) of the relay are connected to a common battery and/or, due to long lead lengths,common mode transient overvoltages ofa relatively high value may appear on the supply leads and differential mode voltages mayarise from switching in other circuits connected to the same battery or supply source;
b) the input energising circuits of the relay are connected to current and voltage transformers;
c) the output circuits are connected to a load by long leads，with the result that common mode transient voltages of a relatively high value may appear at the output terminals.
Category lv
Category lV applies where measuring relays and protection equipment are likely to besubjected to high levels of transient voltage，for example due to not properly shieldedconnection cables.A further example is the direct connection to primary circuits or any otheruse close to the origin of installation.
4.2.2.2Selection of rated impulse voltage
For relays directly energised via current and voltage transformers and for circuits directlyconnected to the station battery supply, an equivalent derived line-to-neutral voltage of 30o vapplies.
4.2.2.3 lmpulse voltage insulation coordination within the measuring relays
For parts or circuits within the relay which are significantly influenced by external transientovervoltages, the rated impulse voltage of the relay applies. Transient overvoltages which canbe generated by the operation of the measuring relay shall not influence external circuitconditions beyond what is specified in 4.2.2.4.
For other parts or circuits within the relay which are specifically protected against transientovervoltages so that they are not significantly influenced by external transient overvoltages,the impulse withstand voltage required for insulation is not related to the rated impulsevoltage of the relay but to the actual conditions for that part or circuit.
4.2.2.4Switching overvoltages generated by equipment
For equipment capable of generating an overvoltage at the relay terminals，for exampleswitching devices, the rated impulse voltage implies that the equipment shall not generateovervoltages in excess of this value when used in accordance with the relevant standard andinstructions of the manufacturer.Otherwise，the user shall provide measures to limit theimpact of the switching overvoltages.
NOTE The residual risk that voltages in excess of the rated impulse voltage can be generated depends on thecircuit conditions.
4.3Time under voltage stress
lt shall be assumed that, for measuring relays and protection equipment, continuous stress over along period exists. Minimum creepage distances shown in table 4 refer to this condition.