The Introduction to IEC 950 states, “It is normal to provide two levels of protection for OPERATORS to prevent electric shock caused by a fault.”
A colleague states, “The basic principles for protection against electric shock in IEC standards is to provide two measures of safety. This means, if one fails, then the product or installation still is safe.”
These phrases have been used for many years in many other publications. What is a “level of protection”? What are “measures of safety”? Why do they apply only to electric shock and not to other injuries?
I’m afraid that these phrases don’t mean much to me. I don’t know what a “level” is as used in this context. Likewise, I don’t know what a “measure” is. These words are vague and abstruse in these contexts. We must discuss something much more concrete if we are two understand the idea of two “levels” or two “measures.”
Let’s examine various sources for electric shock. If we consider the common flashlight battery, we find that we are dealing with an ELV (extra-low voltage) source. As a general rule, ELV is defined as a value of voltage which is not likely to render an electric shock. The value of the voltage is the protection against electric shock. Because the voltage source is a battery, there is no means by which the voltage can exceed ELV as a result of failure of the battery. No other protection against electric shock need be provided. The battery terminals can be accessible.
Therefore, in the case of the battery, there is but one “level” or “measure” of protection. We can now generalize this idea. Any conductive part whose potential does not exceed ELV is “safe” because it is ELV.
For the purposes of equipment construction, the world can be divided into two parts, ELV and non-ELV. Every conductive part of the equipment can be assigned into one of two classes. A grounded part is ELV. An isolated conductive part is ELV. A 3.2-volt, or 5-volt, or 9-volt, or 12-volt circuit is ELV.
In these examples, protection against electric shock is provided by ELV alone. There is no second “level” or “measure.”
Next, let’s consider non-ELV sources such as the mains voltage. A non-ELV source is defined as a source which is likely to render an electric shock. Electric shock is an insidious event. An insidious event is one that you cannot sense until the event occurs. You cannot sense proximity to an electric shock hazard.
Recall when you have received an electric shock. Most likely, the experience of electric shock was an unexpected event, was a surprising event.
On the other hand, the hazard of being cut by a knife is not an insidious event. It is not an insidious event because you can sense (see) the hazard before it occurs. You can sense proximity to the hazard.
Most people become very anxious about hazardous situations which they cannot sense. This is so because if they cannot sense a hazard, then they cannot initiate any means to avoid the hazard. The situation is out of their “control.”
A good example of an insidious hazard is the ionizing radiation emission of a nuclear power plant. Ionizing radiation cannot be sensed prior to its causing an injury. Therefore, ionizing radiation is insidious. Since we cannot sense a radiation leak, we cannot control our safety. Consequently, we demand extensive safety construction for nuclear power plants.
On the other hand, a good example of an insidious hazard converted to a non-insidious hazard is natural gas. The odor added to natural gas allows us to sense the presence of gas before its concentration accumulates to a dangerous level. In the event of a gas leak, we can sense the leak and we can control the situation.
Finally, a good example of a non-insidious hazard is the burning of a candle in our home. We can see the flame, and we can control its location such that the flame is unlikely to cause a fire. Even in the event of the candle igniting another fuel, we can usually control the situation before it gets to a non-controllable magnitude.
So, for most non-insidious hazards, we do not require specific safety construction. However, for most insidious hazards, standards require the situation to be safe not only during normal operation, but also in the event of a single fault. Where the consequences of the hazard are extreme, we require the situation to be safe in the event of multiple faults. Examples include the field of nuclear power, and the field of intrinsic safety.
For non-ELV sources, standards require insulation be interposed between such circuits and ELV parts, whether those ELV parts are grounded, are isolated, or are secondary circuits. Furthermore, because electric shock conditions are insidious, standards require the construction to account for failure of
Note that the first constructional requirement that gives protection against electric shock is that insulation be interposed between the non-ELV source and the ELV part.
Regardless whether we are considering an overhead power line or a coffeemaker, the constructional requirement is that insulation be interposed between the non-ELV source and any other conductive part or the human body. In the case of an overhead power line, the insulation is primarily air, with solid insulation giving support to the wires. In the case of a coffeemaker, the insulation is comprised of both solid insulation and air insulation.
Let’s now look at the issue of insidiousness. The outward manifestation of an operating circuit is its output in a form which a human can sense. This implies an output which can be detected by one of our five senses, seeing, hearing, feeling, smelling, and tasting.
Normal operation of many electrical products results in something we can see, feel, or hear. Through these senses we can deduce that an electrical device is energized.
But, this is not always the case. Consider the overhead power line. How do you know if the power line is energized?
(Several winters ago, I was driving across the Oregon desert during a severe cold following a moderate snowstorm. I passed under two sets of big, overhead power lines, suspended from two sets of steel towers. The first set of power lines had snow and ice on the wires. The second set did not. Which set of power lines was surely energized?)
In most cases, there is no outward sign that a power line is energized. If the power line should fail and fall to the ground, how would we know if it was energized (assuming no sparks or other fireworks)?
If a coffeepot is not operating, there is no outward sign that it is energized. Even if it is plugged into an outlet, there is no outward sign that the outlet is energized. If the insulation of the coffeepot were to fail, how would we know?
These situations are insidious. They appear to our senses to be safe, but are not. So, our safety standards demand that products be safe from electric shock even in the event of failure of the insulation that was providing protection against electric shock.
There are three construction schemes for providing protection against electric shock in the event of failure of insulation:
- Equipotential bonding construction.
- All-insulated construction.
- Double insulation.
- Reinforced insulation.
- Automatic disconnection of the source
- Over current device.
- Ground fault circuit interrupter.
- Immersion detection circuit interrupter.
What about SELV (Safety Extra Low Voltage)? SELV is a special case of ELV. The special case is that the ELV is derived from a non-ELV source.
First, the ELV source, being derived from a higher, non-ELV source, must be maintained as ELV. Usually, this is determined by the turns-ratio of a transformer, or by the junction of a photo-transistor, or some other voltage-determining device. Usually, failure of voltage-determining characteristics of this device is ignored.
Secondly, the ELV must have insulation interposed between it and its higher, non-ELV source. The construction must account for failure of that insulation.
So, SELV actually has at least three and possibly four parameters that must be evaluated in its construction. First, the value of ELV. Second, the insulation between the ELV and the higher non-ELV. Third, the consequences of failure of that insulation. The fourth possibility is the evaluation of a fault that might increase the circuit value to greater than that of ELV.
My point is that the expression “two levels” or “two measures” is rather vague and abstruse. A better expression is that protection against electric shock is provided both for normal operating conditions and for the case of an insulation fault.
Promulgating the idea of “two levels” or “two measures” can lead to ignoring other factors that determine electric shock.
By the way, we apply the same principle to the issue of electrically-caused fire. We determine that the product will not ignite itself, or cause ignition of nearby materials, under both normal operating conditions and in the event of a failure.
Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations.