Altitude: Why it matters
Working with electricity is dangerous, so safety equipment, such as clothing, gloves, boots, etc., is critical in helping protect electrical workers from potential accidents. However, a less obvious safety concern is the design of the test equipment being used to maintain the electrical system.
The International Electrotechnical Commission (IEC) 61010 standard covers safety requirements for electrical equipment for measurement, control, and laboratory use. It includes requirements for testing and measuring circuits and for instruments.
The Measurement Category (CAT) rating of the instrument shows you where in the system the instrument has been designed to be used safely. The higher the rating, the greater the creepage and clearance distances. An often overlooked factor is the altitude where testing is being done. Testing at sea level is not the same as testing in the mountains. This paper offers insights into the safety implications of high altitudes.
Clearance distance refers to the shortest path in the air between two conductive parts that provides sufficient insulation. It is different from creepage distance, which measures the shortest distance along the surface of the insulating material. The clearance can equal but can never be longer than the creepage distance.
Maintaining proper clearance is a critical part of safe printed circuit board (PCB) design. PCB and component layouts must include sufficient safety spacing distances to prevent high voltage arcs or breakdowns between conductors and electronic components, and to protect the device and the user.
The density and insulating strength of air is quite good at sea level, meaning that the required clearance for specific CAT ratings is minimised. Unfortunately, the insulating strength of air changes as the altitude changes.
Paschen’s Law, named after German physicist Friedrich Paschen, is an equation that gives the voltage necessary to start an electric arc between two electrodes in a gas as a function of gas pressure and distance between the electrodes. For a given gas, the voltage is a function only of pressure and gap length. As pressure falls, the breakdown voltage for a fixed gap also falls. Under these conditions, the gap must be widened to maintain a fixed breakdown voltage. This law becomes relevant in instrument design because air is a gas.
The impact of altitude
Air is used as an electric insulating medium in the design of electrical equipment. Atmospheric pressure can be defined as the total weight of air over a unit area at any given elevation. The density and strength of air are very good at sea level. Air gets thinner at higher altitude and therefore becomes less of an insulator. As elevation increases, the amount of air over that unit area decreases. Atmospheric pressure decreases as altitude increases, which reduces the dielectric (insulating) strength of the air.
At lower air pressure, there is less insulation between electrical conductors, which leads to a greater chance of electrical arcing. The reduced pressure causes the air to break down more easily, leading to ionisation, which makes it conduct electricity more easily.
The following chart from The Engineering ToolBox (www.EngineeringToolBox.com) shows the change in atmospheric pressure as elevation increases:
The required clearance distance to ensure safety increases as altitude increases because the insulating properties of the air are decreased. The following table (taken from IEC 62368-1, Table 22) shows the multiplication factor for clearance based on the altitude.
|Normal barometric pressure (kPa)
|Multiplication factor for clearance
Anticipated work elevation becomes a consideration when designing (and when buying) electrical test equipment. IEC 61010-1:2001 specifies operation at or below 2000 meters. An instrument with a specific CAT rating at 2000 meters or less may not meet that CAT rating at 3000 meters or higher.
Understanding the environment where test equipment will be used is critical to selecting instruments that will be safe. Work altitude or elevation is no different. Most of the world sits below 2000 meters, but there are still a significant number of people who work with electricity above this elevation; La Paz, Bolivia has an average elevation of 3689 meters, the Escondida Copper Mine in Chile (the world’s largest copper mine) sits at 3010 meters, and the Antamina Copper mine in Peru is at 4200 meters. These are just a few examples. Most ski resorts, by their nature, sit above 2000 meters.
It is important that, as a user of electrical test equipment, you check the altitude associated with the CAT rating. Remember that the standard is 2000 meters. If the test instrument is going to be used at high altitude, you must be aware of the impact on the required clearances.