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Overvoltage Category I to IV: Where the Instrument Sits in the Supply Chain
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Overvoltage category, sometimes called installation category, classifies the location of the equipment relative to the origin of the installation. Equipment connected near the service entrance sees larger transients than equipment downstream of a transformer, and the standard requires insulation to withstand the transients appropriate to the location.
The four categories
Category IV covers the origin of the installation, such as a service panel or a meter head. Category III covers fixed installations and distribution boards. Category II covers appliances and portable equipment connected to a fixed installation, which is where most bench instruments belong. Category I covers equipment connected to a circuit with measures limiting transient overvoltage, such as a protected electronic circuit.
How it changes the numbers
The category, together with the working voltage and the insulation type, selects a row and a column in the clearance table. Moving from category II to III at mains voltage increases the required clearance by a step, which on a tightly packed PCB can be the difference between a layout that passes and one that needs rework.
The common error
Declaring category II for an instrument whose input terminals are intended to be connected directly to a distribution circuit. The declaration is convenient for the design and wrong for the use. The standard expects the declared category to match the way the instructions tell the user to connect the equipment.
