Product Safety Newsletter – March/April 1990
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.
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. (See IEC 601-1 Second Edition, Sub-clause 57.6, and IEC 950, Sub-clauses 2.7.1 and 2.7.3.)
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.
The Purpose of a 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 I*I*R 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.
Such overheating might raise the temperature of nearby materials to their ignition temperature and result in a fire.
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.
Building Protection
In an electrical distribution system, whether in a building or in a product, automatic prevention of overcurrent conditions is required whenever the wire size is reduced. When wire size is reduced, the cross-sectional area is reduced, and the resistance is increased.
Overheating of wire is due to I*I*R power dissipation in the wire resistance. If I remains constant, and R increases (due to decreased wire size), then power dissipation increases and the wire overheats.
Therefore, when wire size is reduced, it is necessary to reduce the maximum I with a fuse.
In the event of insulation failure between a conductor and ground, the ground return system provides a controlled current pathway by which the current is returned to its source. If the fault impedance is sufficiently low, the fuse will clear the circuit and prevent overheating of the supply and ground return conductors.
(Note that if the fault impedance is high, and the current through the fault does not exceed the overcurrent device rating, then the conductors are not subject to over-heating and the situation is acceptable insofar as the purpose of the fuse. However, power will be dissipated in the fault and may result in a fire or other hazard; we will not explore this situation.)
(The ground return system together with the fuse may also play a role in the prevention of electric shock. I have already discussed this role in a previous column.)
Let’s compare the requirements for the ground return circuit with the characteristics of the different ground return systems used throughout the world.
Operation of a fuse in the event of an insulation fault to ground necessarily requires the ground return circuit impedance to be about the same as that of the supply circuit impedance.
In North America, the ground wire and the neutral wire are the same size and are connected to the same ground rod. This gives reasonable assurance that the ground return circuit impedance is about the same as the supply circuit impedance.
In many European installations, the ground wire and neutral wire are the same size, but are connected to their own independent ground rods. This construction places the earth impedance in series with the ground return circuit impedance. Therefore, in such systems, the ground impedance is necessarily greater than the supply circuit impedance.
While every effort is made to assure a low impedance between the two ground rods, occasionally the earth impedance is too high to cause sufficient current to blow the installation fuse. This fault-condition situation is not subject to overheating, but may result in electric shock conditions.
When the earth impedance between the two ground rods is too high to cause the fuse to operate, then a potential difference exists between those two ground rods. If we assume the worst-case where the fault current is equal to the fuse rating, then virtually the entire supply voltage appears between the two ground rods. If the building metal is connected to the ground wire, then, within the building, there is no potential difference between grounded parts and there is no shock hazard — even though at the ground rod there is a potential, gradient nearly equal to the supply voltage.
To prevent electric shock under these conditions, some European authorities require permanent installation of a “residual current circuit breaker” (RCCB) or “earth leakage circuit breaker” (ELCB) on the load side of the fuse (in the building installation). These devices are electromagnetic versions of the North American GFCI (ground fault circuit interrupter). The RCCB or ELCB units open the circuit when the difference between the phase current and the neutral current exceeds about 5 to 20 milliamperes. For all practical purposes, the RCCB or ELCB act as fuses for ground faults-regardless of the fault impedance.
Now let’s look at the situation when the power distribution system is extended to the equipment by means of a plug and socket and a length of power cord.
Usually, the power cord wire size is smaller than that of the wires supplying power to the socket.
As mentioned previously, whenever the wire size is reduced, a fuse should be required to protect the smaller wire size from overheating. The UK has addressed this situation by requiring a fuse in the plug.
In North America (as opposed to the UK), the fuse in the installation is supposed to protect not only the installation wiring, but also any plugs and cords connected to any receptacle. This requirement is independent of whether or not a fuse is provided within the equipment.
A fuse, being a series element in a circuit could be applied anywhere in the circuit (at the source, at the load, or in the return wire) and still do the job. However, the fuse must be located at the source and in the “hot” leg if it is to provide protection for all possible faults. As a general rule, a fuse should not be used to provide protection of wires and other components on the supply side of the fuse. Therefore, a fuse in the cord-connected product does not provide protection for faults in the cord, line filter, or power switch (where located on the supply side of the fuse).
In North America, wire sizes for power cords, including extension cords, are selected to always be capable of blowing the 15 or 20 amp building fuse in the event of a steady-state short-circuit at the end of the power cord.
The power cord wire size together with its insulation rating must have a sufficiently low impedance to withstand the overheating of the short-circuit until the 20-amp circuit breaker clears the circuit. (Cord-connected electrical heating appliances often have high temperature insulation on their power cords to account for steady-state high current conditions.) To meet this criterion, the minimum wire size for flexible cords is AWG 18, except for specific applications, in which case there are extensive insulation robustness tests designed to preclude insulation failure.
Ampacity ratings of cords and cordsets are given in UL 817, Table 90. These ratings are for normal conditions.
Inside a Product
Thus far, we have been talking about the building power distribution, and protecting wires from overheating in the event of an overcurrent condition. For cord-connected products, electric power distribution stops and electric power utilization begins at the load end of the power cord. When we get inside a cord-connected product, we are no longer necessarily dealing with I*I*R power dissipation in the wire resistance or in the contact resistance of switches and connectors as being the only sources of heat for a fire. Within cord-connected equipment, we have line filters, transformers, and many other circuit components which may be subject to over-heating from E*I power dissipation. Now we must consider both I*I*R and E*I power dissipation.
And, we must consider both insulation and circuit component faults.
To protect against overheating, we must first identify those parts which could dissipate power and which therefore could overheat. Only those parts which can dissipate power can overheat.
Let’s look at the common 50-60 hertz transformer: The ideal transformer dissipates no power. But transformers are wound with copper wire having a finite resistance; power is dissipated in this resistance and some heating results. Transformers are made with imperfect magnetic cores; more power is dissipated in overcoming core losses and more heating results. Normally, the heating from these sources is relatively low.
What faults can cause the transformer to overheat?
Let’s first consider the primary winding. Here, if the magnet wire insulation should fail, some small proportion of the number of turns would be shorted, and power would be dissipated in those shorted turns. The current would increase in proportion to the number of turns shorted which can rarely be a very large number. (The number of turns that could be shorted depends on the transformer construction.)
With shorted turns, whether primary or secondary, the current increases and the E*I power increases. The transformer heats up. The resulting increase in the phase-to-neutral current is not likely be enough to cause the building fuse to open.
Insulation and component failures at the secondary output terminals can also cause transformer overheating without causing the building fuse to open.
Two kinds of output terminal loading simulate the worst-case transformer overheating. One load is the output short-circuit. This maximizes I*I*R heating within the transformer. The other load is the output maximum power. This maximizes the E*I heating within the transformer.
If the heating resulting from shorted turns or excessive loading could cause the failure of insulation or a fire, then some kind of automatic safeguard must be employed.
For many transformers, one or more fuses is an acceptable automatic safeguard. (In some cases of multiple-winding secondaries, both the primary and the secondaries must have their own fuses.) The fuse value is selected at some value greater than maximum normal load, and less than that load which produces a potentially damaging temperature.
Now, let’s take another look at those fuse-less transformers that are acceptable under CSA and UL standards, but not acceptable under IEC standards.
For small transformers where rated input current is a fraction of an ampere, the difference between rated input current and fault current for an unacceptable temperature may not be high enough to find a fuse which will not blow under normal current but will blow under abnormal current. In addition, low current fuses tend to have fragile elements and are subject to failure due to mechanical shock.
In these cases, it is common to use a thermal switch as a safeguard against excessive temperature. Indeed, for small transformers, a thermal switch is a better safeguard against excessive temperature than a fuse. For compliance with IEC 601-1 and with IEC 950, one would need to safeguard the transformer with a thermal switch, and then add a fuse. (Both IEC 601-1 and IEC 950 require that the transformer be protected against overheating under overload or short-circuit conditions; see IEC 601-1, Sub-clause 57.9.1 and IEC 950, Appendix C.) For a small transformer, the fuse is useless but required for compliance.
What about the requirement that, in grounded products, a fuse must be provided in both poles of the supply? I’ve already discussed in the June 1988, Product Safety Newsletter (republished in the February 2014 issue of In Compliance) that double fusing does not compromise the safety of the product and should be permitted.
Clearly, automatic overcurrent or over-temperature safeguards are necessary to prevent electrically caused fire.
The building fuse safeguards the building wiring and power cords for both phase-to-neutral and phase-to-ground overcurrent situations.
The local product fuse safeguards the product parts for local E*I and I*I*R overheating situations.
At the load end of a power cord, a fuse or other device safeguards against excessive temperature due to power dissipation in line filters, transformers, and many other circuit components which may be subject to overheating. All of these kinds of components are connected between the poles of the supply. A single fuse adequately safeguards against local overheating, regardless of which pole is wired.
Meanwhile, the building fuse adequately safeguards against overcurrent in the event of phase-to-ground insulation failure — whether in the building, the power cord, or in the product.
With the exception of line filter capacitors (which are rarely provided with fuse safeguards), there are no power dissipating components connected from pole to ground. Both poles of the mains circuits must be everywhere insulated from ground. Assuming there is no wire size reduction on the load side of the fuse, there is nothing to safeguard against overheating in the event of an insulation failure to ground (except, maybe, the insulation itself) — even in the event of polarity reversal.
Consequently, the building fuse prevents overheating of wires in the event of failure of insulation between the phase conductor and ground. (Note that this is true regardless of plug polarity: the phase conductor in the product is the wire that is connected to the building phase conductor via the plug and not necessarily the wire with the fuse.)
And, the product fuse prevents overheating in the electric power utilization components within the product. (Note that this is true regardless of plug polarity: electric power utilization in the product occurs between the two poles of the supply.)
In this article, I have shown that fusing is not the only means for safeguarding against excessive temperature within products. Indeed, for transformers, at least, fuses may not provide an adequate safeguard.
I have also shown that, while double-fusing should be permitted, double-fusing should not be required as double-fusing does not provide any safeguarding above and beyond that of a single fuse. (In the case of an insulation fault to ground, where the building ground is connected to a ground rod independent of the ground wire, and where the ground impedance is too high to create overcurrent sufficient to operate the building fuse, a local fuse may reduce conditions for electric shock.)
From a practical point of view, somewhere in the neighborhood of 4 to 6 amperes and lower, no automatic prevention of overcurrent conditions is required whenever the wire size is reduced. The power dissipated in the wire or contact resistance, together with the power per unit volume and the increase in resistance due to the heating, usually is not capable of an overheating condition.
Mr. Nute is a Life Senior Member of the IEEE, a charter member of the Product Safety Engineering Society (PSES), and a Director of the IEEE PSES Board of Directors. He was technical program chairman of the first 5 PSES annual Symposia and has been a technical presenter at every Symposium. Mr. Nute’s goal as an IEEE PSES Director is to change the product safety environment from being standards-driven to being engineering-driven; to enable the engineering community to design and manufacture a safe product without having to use a product safety standard; to establish safety engineering as a required course within the electrical engineering curricula.
