Technical Articles

What is SELV circuit?

SELV circuit, full name is safety extra-low voltage circuit, refers to the electric shock protection of an ungrounded system.

Safety ultra-low voltage circuit is a secondary circuit that has been properly designed and protected so that, under normal conditions or single fault conditions, the voltage between any two accessible parts and between the protective earthing terminals of any accessible parts and equipment (only for Class I equipment) does not exceed the safe value.

1. Generally, 42V is used for handheld power tools; 36V, 24V for safety lighting in general places; 12V for lighting and portable lamps in particularly humid places and metal containers; 6V for underwater lighting. 【Regardless of alternating current (AC) and direct current (DC)】

2. For three-phase power supplies, no more than 29 volts between the wire and the center line. For power tools, these two voltage limits are no-load values.

3. In China, the safety voltage rating is specified to be 42, 36, 24, 12, 6 volts. When the voltage used by electrical equipment exceeds the safe voltage, protective measures to prevent direct contact with the charged body must be taken according to the regulations.

4. The IEC standard stipulates that under normal conditions, the upper limit value of the contact voltage is 50 volts AC or 120 volts DC without ripple. In special cases, the upper limit of the contact voltage is 25 volts AC or 50 volts DC without ripple.

5. In circuits isolated from the mains by a safety isolation transformer or converter with independent windings, the rms value of AC voltage between conductors or between either conductor and ground does not exceed 50 (Note 1: DC voltage is under consideration. NOTE 2: It is assumed that any transformer or converter operating at its rated supply voltage should not exceed this voltage limit at full or no load. )



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