One manufacturer says its DMM is a Category II multimeter with isolation. Another says its DMM is a Category I unit with double insulation. What does all this terminology mean? If both DMMs are rated at 250 V, is there a difference? To cut through all this marketing hype, you need to know where these terms come from and what they mean.
Who Says What Is Safe?
Whenever safety is an issue in a product, usually there is a standard stating what is safe and what is not. High voltages are no exception. Both the European Union (EU) and the Underwriters Laboratory (UL) have issued standards covering safe design of high-voltage instruments.
In 1973, the EU issued the Low-Voltage Directive (72/23/EEC). This document defined voltages requiring special considerations for safe use in an electrical device. These levels range from a minimum of 50 VAC or 75 VDC to a maximum of 1,000 VAC or 1,500 VDC. On Jan. 1, 1997, the Low-Voltage Directive was included as a mandatory requirement for the CE Marking scheme.
There are approximately 200 individual safety standards harmonized (approved for use to demonstrate compliance) to the Low-Voltage Directive. The relevant standard for instrument manufacturers is EN 61010: Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use.
EN 61010 is a little stricter than the Low-Voltage Directive. It says dangerous voltages are 30 VAC rms or 60 VDC. In addition to high-voltage design requirements, EN 61010 includes safety design constraints, such as flammability or heat. Instrument manufacturers must meet all the specifications in EN 61010 to receive the CE Marking.
Two other standards are very similar to EN 61010: the International Electrotechnical Commission (IEC) 1010 and UL 3111. IEC 1010 is the precursor to EN 61010. The EU adopted it and renamed it EN 61010.
UL 3111 also is a child of IEC 1010. The UL made some modifications to IEC 1010 and then adopted it as UL 3111. This new, strict UL standard replaces the older, more lenient UL 1244 for measurement, control, and laboratory instruments. For new designs, instrument manufacturers must meet all the specifications in UL 3111 to receive a UL listing.
There are many electrical safety issues that manufacturers must keep in mind to make their products compliant with these standards. For simplicity, we will refer only to the original standard, IEC 1010. However, all of the observations about IEC 1010 apply to UL 3111 and EN 61010 as well.
What Is Isolation?
Let’s look at one of the major ways instrument manufacturers provide high-voltage safety—isolation. Isolation physically and electrically separates two parts of a circuit, but allows the two sides to interact.
Isolation is achieved by using three main coupling methods: Optocouplers (light), transformers (magnetic flux), and capacitive coupling (electric field).
Isolation is popular with instrument manufacturers because it provides several advantages in both safety and measurement accuracy:
It breaks ground loops.
It improves common-mode voltage rejection.
It allows the sides of the circuit to be at different voltage levels. For example, one side of the circuit can be at a low-voltage level while the other side is at a hazardous voltage level.
For isolation to be safe, it must have high-integrity isolation components and a safe insulator barrier. Examples of this insulation barrier are a piece of plastic, a keep-out space in a PCB, or an air gap.
How Much Insulation Makes a Product Safe?
The amount of insulation required in the isolation barrier depends on several factors:
Working Isolation Voltage—Larger isolation voltages require more insulation.
Transient Voltage—Insulation strong enough to withstand the normal working voltages of the circuit can break down under large transients. As a result, larger transients will require more insulation.
Air Pollution—Insulation can be shorted by contaminants in the air. Dirtier environments require more insulation.
Single-Fault Current Path—If the insulation breaks down, can the shorted current go through a human body? If so, a larger amount of insulation is required.
The IEC has covered these issues in Section 6 of IEC 1010. It has defined terms like overvoltage categories, pollution degrees, and double insulation.
Overvoltage Categories
The IEC defined the term overvoltage category to address transient voltages. Category IV devices can handle the largest transients relative to the normal working voltage. Category I devices can accommodate only small transients. For example, a 250-V Category IV device can handle transients of up to 6,000 V. The highest transient a 250-V Category I device can withstand is 1,500 V.
Table 1 shows the transients each category allows.1 Category IV transients are not listed here because IEC 1010 does not cover this category. Figures 1 and 2 illustrate how category ratings affect what transients an instrument can withstand.
Both the 250-V Category I meter and the 250-V Category II meter can measure the base levels of these waveforms. However, only the Category II meter can safely withstand the transient in Figure 2.
Here is how the IEC defines its overvoltage categories:2
Category IV—used at the origin of the installation, such as electricity meters and primary overcurrent protection equipment. Although the IEC has defined this category in other documents, IEC 1010 does not cover this overvoltage category.
Category III—in fixed installations and for cases where the reliability and the availability of the equipment are subject to special requirements. Examples include switches in fixed installation and equipment for industrial use with permanent connection to the fixed installation. Equipment intended to measure the voltage levels of these fixed installations must be rated at this overvoltage category.
Category II—energy-consuming equipment to be supplied from the fixed installation, such as appliances, portable tools, and other household and similar loads. Equipment intended to measure the voltage levels of these fixed installations must be rated at this overvoltage category.
Category I—for connection to circuits in which measures are taken to limit transient overvoltages to an appropriately low level; for example, protected electronic circuits.
Equipment rated at Category I cannot measure voltage levels at Overvoltage Category II or III.
What does all this mumbo-jumbo mean? Let’s look at the example of the house in Figure 3. Category IV voltage transients are present on the power lines outside the house. These power lines are fed into a transformer on the pole.
The power signal coming out of this transformer is Category III. This signal then goes through the fuse panel and the distribution network of the house to provide a Category II level at the wall outlet. To get to Category I levels, additional transient protection, such as an isolation transformer, must be provided after the wall socket.
Degrees of Pollution
IEC 1010 specifies different types of pollution environments. Harsher environments require more insulation. As an alternative to increased insulation, the designer can create a cleaner microenvironment for the circuit by using enclosures, encapsulation, or hermetic sealing.
Four pollution environment categories have been defined in the IEC 664-1 Specification:3
Pollution Degree 1
No pollution or only dry, nonconductive pollution occurs. The pollution has no influence. One example is a circuit in a hermetically sealed box, such as an IC chip. No air can come into the box to introduce condensation or conductive particles.
Pollution Degree 2
Only nonconductive pollution occurs. Occasionally, temporary conductivity caused by condensation must be expected, such as a circuit used in an office environment. The circuitry inside a computer would fall under this category.
Pollution Degree 3
Conductive pollution or dry, nonconductive pollution occurs which, as expected, becomes conductive due to condensation. Circuitry exposed to outside air but without contact with precipitation would be one example. A garage-door opener would fall under this category. Although the IEC has defined this pollution degree in other documents, IEC 1010 does not cover Pollution Degree 3.
Pollution Degree 4
The pollution generates persistent conductivity caused by conductive dust, rain, or snow, such as an exposed outdoor control box for a water pump. Although the IEC has defined this pollution degree in other documents, IEC 1010 does not cover Pollution Degree 4.
Insulation Types
In any isolation scheme, a certain amount of insulation is required to create the isolation barrier. IEC 1010 calls this basic insulation. If a breakdown in the insulation could cause dangerous current to flow through a human body, basic insulation is not enough protection.
IEC 1010 provides several insulation improvement options, two of which are double insulation and reinforced insulation. Double insulation is basic insulation plus some supplementary insulation, such as another basic layer. If the basic insulation breaks down, the supplementary insulation keeps the user safe. Reinforced insulation serves the same purpose as double insulation except that the basic and the extra insulation cannot be tested separately.
What Do These IEC Definitions Mean for You?
Knowing the IEC definitions allows you to understand what your present measurement instruments can do and what you need to buy in the future. For example, a 250-V Category I DMM is not rated to measure standard wall-socket voltages. The DMM is not designed to withstand the transient voltages on the power line. However, a Category II 250-V DMM measures wall-socket voltages.
When measuring high voltages, safety is a big concern. When using your current equipment or buying new, pay attention to more than just the working voltage rating. Make sure your equipment meets the UL, CE, or IEC standards you need. That way you’ll be assured that the high voltage goes into your measurement circuit instead of you.
References
1. Table J.1, IEC 1010-1 Specification, International Electrotechnical Commission, First Edition, 1990-9, p. 181.
2. IEC 664-1 Specification, International Electrotechnical Commission, First Edition, 1992-10, Section 2.2.2.1.1, p. 25.
3. IEC 664-1 Specification, International Electrotechnical Commission, First Edition, 1992-10, Section 2.5.1, pp. 33-34.
About the Author
Matt Duff, who joined National Instruments in 1997, is a hardware engineer at the company. He holds a B.S. in electrical engineering from Texas A&M University.
Michel Haddad is R&D Group Manager at National Instruments. He has been employed by the company since 1990. Mr. Haddad received a B.S. in electrical engineering from George Washington University and an M.S. in electrical engineering and computer science from the Massachusetts Institute of Technology.
National Instruments, 11500 N. Mopac Expressway, Austin, TX 78759, (512) 794-0100, e-mail: [email protected].
Copyright 1998 Nelson Publishing Inc.
November 1998
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