Low voltage lighting has gained popularity over the last several years due in large part to the generally safer operation it presents. In many cases, low voltage lighting is indicated for use in specific locations according to OSHA regulations and NEC recommendations. Low voltage lighting, that is lighting below 30VDC, is generally considered safer than typical 120-277 VAC lighting due to the fact that the potential for serious or injury of death from electrocution is greatly reduced. This is not to say however that there are no risks associated with the use of low voltage lamps, and it is important to note that although low voltage lamps may impart a greater sense of safety, complacency must still be avoided.
Typical work site lighting operates with voltages in the 120 to 277 VAC range. On large sites and industrial environments, voltages are often 240, 280, or range as high as 480V although voltages such as this are rarely associated with use in lighting systems. While using standard voltage lighting (110-120VAC) is usually more convenient and practical, it brings with it added risks in the form of electrical shock should a fault or short condition occur.
Although injury is possible with either AC or DC current, AC current is considered more dangerous and twice as likely to cause serious injury depending on comparable voltages. For instance, a current as low as 1 mA of AC at 60 Hz can be felt by a person, while it takes at least 5 mA of DC to be perceptible. 30 mA of AC is enough to cause fibrillation leading to cardiac arrest, while it takes 300–500 mA of DC to cause fibrillation.
The Nature of the Current
The reasons for this are due to the nature of the current types involved. AC stands for “Alternating Current”, and as the name suggests, AC current continuously changes direction and passes from a zero to maximum positive value, then to a maximum zero value. AC voltage has a peak root mean square, or “RMS” value, and the maximum value is 1.4 times the RMS. These means that although a standard household outlet might be 120 VAC, its peak maximum value is actually 168 VAC before it passes back down to zero. How fast this occurs determines the frequency of AC current.
DC current on the other hand does not have this alternating nature and is instead known as “Direct Current. Its magnitude does not change and it does not have a frequency, but it can be negative or positive according to the direction of a circuit. The upshot of all this is that the effect on the human body differs according to the type of current it is exposed to. Since AC is alternating and has frequency, when passing through the human body it causes a very rapid series of muscle contractions. DC current in contrast does not alternate and thus causes a single contraction.
In addition, at around only 10 mA AC the human body experiences severe muscle contractions which can cause an inability to let go of an energized object, which is called the "let go threshold". This inability to let go is often responsible for the more serious injuries incurred with contact with AC current as the duration of the contact is increased. DC current on the other hand has no frequency, and when passing through the human body causes only a single contraction that lasts as long as the current is being applied. When contact with direct current strong enough to cause a major involuntary contraction occurs, the person tends to be thrown away from the contact by the force of the contraction, thus reducing the length of exposure.
So what's more dangerous?
What this all boils down to is that alternating current is potentially more dangerous than direct current for a given voltage. Thus, if one were concerned with the potential for electric shock, perhaps in a work environment where wet conditions may be encountered, it would make sense to consider using low voltage lighting equipment. Especially in locations where space is limited and the surrounding area is dominated by metal surfaces and metal structures, reducing the risk of electric shock is critical as workers are usually in direct contact with conductive surfaces at all times.
Low voltage lighting offers protection because the power in the device and power supplying cord is below the “let go threshold” as well as below what is generally considered enough to cause fibrillation through skin contact. Sure, very low voltages below 12 VDC can cause cardiac arrest if applied directly to the heart muscle, but for practical purposes exposure from hand contact for instance will not. Should a worker inside a tank or large conduit break a lamp or a cord becomes damaged and contacts metal surfaces, the surrounding surfaces can become energized, but due to the voltage being so low the potential for serious injury is greatly reduced. The worker will not experience a sustained muscle contraction which will prevent him from removing himself from contact with the energized surface, and even should he experience a sustained contact the energy will be far less likely to cause serious lasting injury.
All of this is not to say that one can thus become complacent because they are using low voltage illumination. Even though the potential for serious electrocution is greatly reduced, there remains enough energy to cause serious shorts, sparks, and heat generation if a fault occurs, all of which can have serious repercussions, particularly in environments deemed hazardous due to the presence of flammable vapors, gases and materials. Because of this, low voltage lighting is still required to have the proper housings, covers and fault protections, and is still required to meet the same standards for explosion proof and intrinsically safe certification as equipment operating from 120 VAC.
Low voltage lighting by virtue of it being less capable of delivering damaging levels of current is an effective way to lessen the hazard posed by potential electrocution. Utilizing low voltage illumination in environments where the chances of contact with conductive surfaces and wet conditions is high can help prevent the incidence of serious accidents and injury. Those considering low voltage lighting should keep in mind the normal protections and certifications associated with lighting must be maintained, and particularly in hazardous locations, compliance through the proper approvals remains mandatory.