Almost every safety standard has requirements addressing the accessibility of certain parts. These take either of two forms:
Hazardous parts shall not be accessible or Accessible parts shall not be hazardous
The single most common device used to fulfill these two requirements is the enclosure. Enclosing hazardous parts within an enclosure renders those parts inaccessible.
Here are the requirements restated as “fulfillments:”
Enclosed hazardous parts are not accessible,
or
Enclosure accessible parts are not hazardous.
Let’s examine in depth how the enclosure makes a product safe. How does making the parts inaccessible make a product safe? The obvious answer is:
We cannot touch hazardous parts.
But, if we remove the enclosure and operate the product then we have:
Hazardous parts which are accessible,
and
Accessible parts which are hazardous.
Some of us do this every day, yet we do not incur injury. Clearly, the enclosure does not provide a safety function as products can be operated safely without the enclosure. When there is no enclosure, what mechanism provides the protection against injury?
The obvious answer is: We do not incur injury because we discern which accessible parts are hazardous and then, voluntarily, choose to not touch those accessible hazardous parts. Note that the requirement for injury is that the part must be touched.
If the part is not touched, then no injury occurs. This implies that there is something interposed between the body part and the hazardous part that provides safety. What is this thing?
Let’s examine the situation of the enclosure as protection against electric shock. We don’t have everyday life household examples of hazardous electrical energy available to touch. But, outside the household, we have some very good examples: overhead power lines.
Overhead power lines are not enclosed. But, they are not accessible to touch because they are mounted high on poles or towers that have no readily available means for climbing. Let’s presume a means is provided such that you can climb the power pole or tower. How close are you willing to approach the bare power line? Would you be willing to approach that power line to the minimum distance such that you are not likely to touch it? Probably not.
For the moment let’s presume the power line is insulated. Now, how close are you willing to approach the insulated power line? Would you be willing to touch the insulation? Probably. What is the thing that is interposed between you and the insulated power line that provides protection against electric shock?
Answer: Insulation
In the case of the insulated power line, insulation is interposed between you and the power line. The insulation is providing the protection against
electric shock.
But, what provides the protection against electric shock in the case of a bare power line? Clearly, if we touch the bare power line we will incur a shock. Conversely, if we do not touch the bare power line, we will not incur a shock. This implies that there is something interposed between the body part and the hazardous part that provides safety. What is this thing?
Answer: Insulation
As in the case of the insulated power line, there must be insulation interposed between you and the bare power line. What is this insulation?
Answer: Air
Air is an electrical insulating medium. Air is the most common, reliable, and cheapest insulator in use today.
The air between you and an energized part provides the insulation that protects you against electric shock, from any voltage.
All parts not enclosed in solid insulation are automatically enclosed in air insulation. The air occupies a volume surrounding the part. In the case of overhead power lines, this volume appears to extend for miles. But, the overhead lines only need some minimum volume around them to be sufficient insulation to provide protection against electric shock. We’ll discuss this later in this paper.
The problem with air as insulation is that it is a fluid. Because it is a fluid, the volume of air providing insulation can be DISPLACED by any solid body, including a part of the human body.
When the particular volume of air that is providing insulation is displaced by a body part the insulation is thereby removed from the hazardous part and a shock or burn can be incurred.
The function of the enclosure (or the pole or tower for power lines) is to preserve the air as an insulator. The enclosure, pole, tower, or fence (around a substation) is a barrier that prevents a body part or other foreign object from displacing the volume of air insulation that is providing the protective function.
The function of the enclosure is NOT to prevent access to hazardous conductors. The function of the enclosure is to PRESERVE (prevent displacement of) the insulation provided by the volume of air surrounding the parts.
The conventional wisdom that preventing accessibility thereby prevents injury is a myth. How much air is required to provide protection against electric shock?
Insulation can be modeled as a parallel circuit comprised of a capacitor, a resistor, and a spark-gap.
By definition, any two conductors separated by an insulating medium constitute a capacitor. In the case of electric shock one plate of the capacitor is the energized conductor, the other plate is the body part. (For the purposes of evaluating electric shock, the body should be thought of as a grounded conductor.)
Insulation has a finite value of resistance. Usually, it is sufficiently high that it can be ignored. As with the capacitor, one terminal of the resistor is the conductor, the other terminal is the body.
Finally, all insulations will break down if the voltage across that insulation is high enough. This is the spark-gap part of the model. As with the capacitor and resistor, one terminal of the spark gap is the conductor, the other is the body.
Insulation is not always insulation. Insulating materials have two states, one being that of an insulator, the other being that of a conductor (when the insulation breaks down). (There are some intermediate states which we will ignore in this discussion.)
While we normally can’t see air, we have all seen evidence of air in both states, as an insulator and as a conductor. Most of the time, we see evidence of air as an insulator, i.e., electrical energy remains in the conductors. When we see an arc, we see evidence of air as a conductor, i.e., electrical energy leaps from one conductor through the air to another conductor.
The line between insulation and conduction is the electric strength of the insulation.
The performance of air as an insulator is clearly depicted in IEC 664-1. The electric strength of air is principally a function of the distance through air. The more air, the higher the electric strength of the insulation.
The worst-case line between air as an insulator and air as a conductor (breakdown) for distances between 0.1 mm up to about 1.0 mm is about 1100 volts peak per mm plus 700 volts peak.
The best-case line between air as an insulator and air as a conductor (breakdown) for distances between 0.1 mm up to about 1.0 min is about 3400 volts peak per mm plus 700 volts peak.
Stated as formula:
Peak breakdown voltage (worst) = (1100)(D) + 700
Peak breakdown voltage (best) = (3400)(D) + 700
where D is the distance between conductors in mm, from 0.1 to 1.0 mm.
(For those familiar with IEC 664, these formulae are for inhomogenous and homogeneous fields. The point of this discussion is that air is an insulator. This discussion is not to discuss the specific parameters of air insulation.)
In answer to the question, how much air is required to provide protection against electric shock, at 1 mm,
air will break down at 1800 volts peak or about
1200 volts rms (worst), and 4100 volts peak or
about 2900 volts rms (best).
At 1 mm, air is always an insulator for all voltages below 1200 volts rms, and always a non-insulator for all voltages greater than 2900 volts rms.
The principal means of providing protection against electric shock is the interposition of insulation between the conductor and the body.
In other words, protection against electric shock is by enclosing with solid insulation or enclosing with air insulation or a combination of both. When using air insulation, a physical barrier such as an enclosure or other device may be employed to prohibit inadvertent displacement of the air insulation.
Working this way about air gives a powerful tool for the design of products. For example, the problem of the hair dryer falling into a bathtub is a problem of water displacing the air insulation. If a hair dryer did not use air insulation, then there would be no hazard when dropped into the bathtub.
On the other hand, if you could build a detector to detect when water displaced the air, then you could automatically disconnect the dryer from the supply voltage, thus providing protection against electric shock. Newer hair dryers use such a device.
Inaccessibility as a means of protection is a myth. Accessibility is nothing more than a measure of whether or not the air insulation can be displaced by a body part.
Copyright 1993 by Richard Nute Originally published in the Product Safety Newsletter, Vol. 6, No. 6, December, 1993
