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The Use of Overcurrent Protection
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Consider the “direct plug-in” Class 2 transformer-power supply commonly used for low-power devices such as calculators, cassette players, and similar products. Consider also the larger “cousin” known in the trade as the “indirect plug-in” Class 2 transformer (where the transformer is too large to hang from the plug, and must, instead, reside on the floor using attached cords for input power and output power). These small transformer-power supply devices have no power switch, and have no fuses in the primary circuit.Get more news about Overload Protection Socket,you can vist our website!
In North America, it is common for these Class 2 transformer-power supply devices to be safety certified for many uses, including medical equipment (CSA 125 and UL 544) and data processing equipment (CSA 220 and UL 478).
However, these same transformer-power supply devices do not comply with either IEC 601-1 (medical) or IEC 950 (data processing) because they have no fuses in the primary circuit. Not only do IEC 601-1 and IEC 950 require fuses, for grounded products they require fuses in both poles of the supply. (The possible exceptions to this requirement will not be discussed in this article.)
This dilemma has given me cause to pause and consider the hazards addressed by the IEC fuse requirements for single-phase plug-and-socket connected equipment.
Let’s begin by discussing the purpose of a fuse, and the function of the building overcurrent protective device as it relates to plug-and-socket connected equipment. Next, we’ll discuss when to use a fuse, and whether to fuse both poles of a single-phase, plug-and-socket connected equipment. We’ll include a discussion on how to select the value of the fuse.
Let’s review the function of a fuse. (In the context of this article I use the word “fuse” as a general term for automatic overcurrent protective device which includes but is not limited to fuses and circuit-breakers.)Fuses are means to automatically disconnect power under overcurrent conditions.What is overcurrent? Overcurrent is any current exceeding the maximum current rating of wires, switches, connectors, etc.
Why are we concerned about overcurrent? Overcurrent results in overheating. And overheating can result in fire.
Overheating is due to IIR power dissipation in the wire resistance or in the contact resistance of switches and connectors. If I increases (overcurrent), then power dissipation increases very rapidly with increasing I (due to the square function in the power equation) and the conductor or contact overheats. When conductors and contacts overheat, their resistance goes up contributing further to increase power dissipation and the situation approaches a thermal runaway situation.
In a lesser situation, the over-heating might melt plastic wire insulation thus providing undesired and uncontrolled current pathways. We assume that this lesser situation results in a hazard, and, therefore, insulation failure due to overheating must be prevented.Disconnection must be automatic because the circuit may not be continuously attended by someone who will manually disconnect the power, and because overcurrent conditions are not necessarily immediately apparent.