There are many issues to consider when measuring high voltage. When specifying a data acquisition (DAQ) system, the first question you should ask is whether the system will be safe. Making high-voltage measurements can be hazardous to the equipment, to the unit under test, and to you and your colleagues. To ensure that the system is safe, you should provide an insulation barrier, using isolated measurement devices, between the user and hazardous voltages.
Table of Contents
- What Is Isolation?
- Isolation Considerations
- Typical Applications Requiring Isolation
What Is Isolation?
Isolation is a means of physically and electrically separating two parts of a measurement device, and can be categorized into electrical and safety isolation. Electrical isolation pertains to eliminating ground paths between two electrical systems. By providing electrical isolation, you can break ground loops, increase the common-mode range of the data acquisition system, and level shift the signal ground reference to a single system ground. Safety isolation references standards have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high voltages and transient voltages from transmitting across its boundary to other electrical systems with which you can come in contact.
Incorporating isolation into a DAQ system has three primary functions: preventing ground loops, rejecting common-mode voltage, and providing safety.
Ground Loops
Ground loops are the most common source of noise in data acquisition applications. They occur when two connected terminals in a circuit are at different ground potentials, causing current to flow between the two points. The local ground of the system can be several volts above or below the ground of the nearest building, and nearby lightning strikes can cause the difference to rise to several hundreds or thousands of volts. This additional voltage itself can cause significant error in the measurement, but the current that causes it can couple voltages in nearby wires as well. These errors can appear as transients or periodic signals. For example, if a ground loop is formed with 60 Hz AC power lines, the unwanted AC signal appears as a periodic voltage error in the measurement.
When a ground loop exists, the measured voltage, Vm, is the sum of the signal voltage, Vs, and the potential difference, 
Vg, which exists between the signal source ground and the measurement system ground, as shown in Figure bellow. This potential is generally not a DC level; therefore, the result is a noisy measurement system, often showing power-line frequency (60 Hz) components in the readings.
To avoid ground loops, ensure that there is only one ground reference in the measurement system, or use isolated measurement hardware. Using isolated hardware eliminates the path between the ground of the signal source and the measurement device, therefore preventing any current from flowing between multiple ground points.
A Grounded Signal Source Measured with a Ground-Referenced System Introduces Ground Loop
Common-Mode Voltage
An ideal differential measurement system responds only to the potential difference between its two terminals, the (+) and (-) inputs. The differential voltage across the circuit pair is the desired signal, yet an unwanted signal can exist that is common to both sides of a differential circuit pair. This voltage is known as common-mode voltage. An ideal differential measurement system completely rejects, rather than measures, the common-mode voltage. Practical devices, however, have several limitations described by parameters such as common-mode voltage range and common-mode rejection ratio (CMRR), which limit this ability to reject the common-mode voltage.
The common-mode voltage range is defined as the maximum allowable voltage swing on each input with respect to the measurement system ground. Violating this constraint results not only in measurement error, but also in possible damage to components on the board.
Common-mode rejection ratio describes the ability of a measurement system to reject common-mode voltages. Amplifiers with higher common-mode rejection ratios are more effective at rejecting common-mode voltages. The CMRR is defined as the logarithmic ratio of differential gain to common-mode gain.
CMRR (dB) = 20 log (Differential Gain/Common-Mode Gain). (Equation 1)
Common-mode voltage is shown graphically in Figure bellow. In this circuit, CMRR in dB is measured as 20 log Vcm/Vout where V- = Vcm.
In a non-isolated differential measurement system, an electrical path still exists in the circuit between input and output. Therefore, electrical characteristics of the amplifier limit the common-mode signal level that can be applied to the input. With the use of isolation amplifiers, the conductive electrical path is eliminated and the common-mode rejection ratio is dramatically increased.
CMRR Measurement Circuit
Isolation Considerations
There are several terms to know when configuring an isolated system:
Installation Category: A grouping of operating parameters that describe the maximum transients that an electrical system can safely withstand. Installation categories are discussed in more detail later.
Working Voltage: The maximum operating voltage at which the system can be guaranteed to continuously safely operate without compromising the insulation barrier.
Test Voltage: The level of voltage to which the product is subjected during testing to ensure conformance.
Transient Voltage (Over-voltage): A brief electrical pulse or spike that can be seen in addition to the expected voltage level being measured.
Breakdown Voltage: The voltage at which the isolation barrier of a component breaks down. This voltage is much higher than the working voltage, and often times is higher than the transient voltage. A device cannot operate safely near this voltage for an extended period of time.
Isolation Types
Physical isolation is the most basic form of isolation, meaning that there is a physical barrier between two electrical systems. This can be in the form of insulation, an air gap, or any non-conductive path between two electrical systems. With pure physical isolation however, we imply that no signal transfer exists between electrical systems. When dealing with isolated measurement systems, you must have a transfer, or coupling, of energy across the isolation barrier.
There are three basic types of isolation that can be used in a data acquisition system:
Optical Isolation
Optical isolation is common in digital isolation systems. The media for transmitting the signal is light and the physical isolation barrier is typically an air gap. The light intensity is proportional to the measured signal. The light signal is transmitted across the isolation barrier and detected by a photoconductive element on the opposite side of the isolation barrier.
Optical Isolation
Electromagnetic Isolation
Electromagnetic isolation uses a transformer to couple a signal across an isolation barrier by generating an electromagnetic field proportional to the electrical signal. The field is created and detected by a pair of conductive coils. The physical barrier can be air or some other form of non-conductive barrier.
Transformer
Capacitive Isolation
Capacitive coupling is another form of isolation. An electromagnetic field changes the level of charge on the capacitor. This charge is detected across the barrier and is proportional to the level of the measured signal.
Capacitive coupling is another form of isolation. An electromagnetic field changes the level of charge on the capacitor. This charge is detected across the barrier and is proportional to the level of the measured signal.
Capacitor
Isolation Topologies
It is important to understand the isolation topology of a device when configuring a measurement system. It can be categorized into electrical and safety isolation. This section discusses electrical isolation. For electrical isolation, parts of a device are separated with isolation barriers that separate the card into sections. Multiple buses service each section individually. The 3 types of isolation can be listed from basic (low level protection) to complete (high level protection) in the order of: Channel to Earth Ground, Bank, and Channel to Channel isolation.
Channel-to-Earth Ground
Channels of the device and the device's Earth ground are electrically isolated from one another. Channel-to-Earth isolation is represented in the figure below. Voltages of the isolated front end (Va-c ) are on the same bus; these voltages are not isolated from one another. Ve,d are on a separate bus and are isolated from the front end. This is the most fundamental type of isolation, this protection is covered by bank and channel-to-channel isolation. Normally, this isolation type is present on NI 9000 series modules and some SCC modules.
Note: For the following diagrams, the diagonal hash marks indicate the isolation barrier, this separates circuitry. The transformer symbols represent electromagnetic isolation used to couple a signal across the isolation barrier by generating an electromagnetic field proportional to the electrical signal.
It is important to understand the isolation topology of a device when configuring a measurement system. It can be categorized into electrical and safety isolation. This section discusses electrical isolation. For electrical isolation, parts of a device are separated with isolation barriers that separate the card into sections. Multiple buses service each section individually. The 3 types of isolation can be listed from basic (low level protection) to complete (high level protection) in the order of: Channel to Earth Ground, Bank, and Channel to Channel isolation.
Channel-to-Earth Ground
Channels of the device and the device's Earth ground are electrically isolated from one another. Channel-to-Earth isolation is represented in the figure below. Voltages of the isolated front end (Va-c ) are on the same bus; these voltages are not isolated from one another. Ve,d are on a separate bus and are isolated from the front end. This is the most fundamental type of isolation, this protection is covered by bank and channel-to-channel isolation. Normally, this isolation type is present on NI 9000 series modules and some SCC modules.
Note: For the following diagrams, the diagonal hash marks indicate the isolation barrier, this separates circuitry. The transformer symbols represent electromagnetic isolation used to couple a signal across the isolation barrier by generating an electromagnetic field proportional to the electrical signal.
Schematic of Channel-to-Earth Isolation.
Bank (Channel-to-Bus) Isolation
Channels of a device are banked (grouped) together to share a single isolation amplifier. Figure bellow represent Bank isolation. In this topology, the common-mode voltage difference between channels is limited, but the common-mode voltage between the bank of channels and the non-isolated part of the measurement system can be large. In other words, individual channels are not isolated, but the channel groups are isolated from one another and earth ground. Bank1, Bank2, Vi, and Vj,k are on separate buses and isolated from one another. This topology is a lower-cost isolation solution because this design shares a single isolation amplifier and power supply.
Channels of a device are banked (grouped) together to share a single isolation amplifier. Figure bellow represent Bank isolation. In this topology, the common-mode voltage difference between channels is limited, but the common-mode voltage between the bank of channels and the non-isolated part of the measurement system can be large. In other words, individual channels are not isolated, but the channel groups are isolated from one another and earth ground. Bank1, Bank2, Vi, and Vj,k are on separate buses and isolated from one another. This topology is a lower-cost isolation solution because this design shares a single isolation amplifier and power supply.
Schematic of Bank (Channel-to-Bus) Isolation.
Bank Topology
Channel-to-Channel
The most robust isolation topology is channel-to-channel isolation. In this topology, each channel is individually isolated from one another and from other non-isolated system components. In addition, each channel has its own isolated power supply. Figure bellow represents channel-to-channel isolation. Va,b, Vc,d, Ve,f, and Vg,h are all on separate buses and are isolated from one another.
The most robust isolation topology is channel-to-channel isolation. In this topology, each channel is individually isolated from one another and from other non-isolated system components. In addition, each channel has its own isolated power supply. Figure bellow represents channel-to-channel isolation. Va,b, Vc,d, Ve,f, and Vg,h are all on separate buses and are isolated from one another.
Schematic of Channel-to-Channel Isolation.
In terms of speed, there are several architectures from which to choose. Using an isolation amplifier with an analog to digital converter (ADC) per channel is typically faster because you can access all of the channels in parallel. A more cost-effective, but slower architecture, involves multiplexing each isolated input channel into a single ADC.
Another method of providing channel-to-channel isolation is to use a common isolated power supply for all of the channels. In this case, the common-mode range of the amplifiers is limited to the supply rails of that power supply, unless front-end attenuators are used.
Another method of providing channel-to-channel isolation is to use a common isolated power supply for all of the channels. In this case, the common-mode range of the amplifiers is limited to the supply rails of that power supply, unless front-end attenuators are used.
Channel-to-Channel Multiplexed Topology
Safety and Environmental Standards
When configuring a DAQ system, you must take the following steps to ensure that the product meets applicable safety standards:
Not all isolation barriers are suitable for safety isolation. Even though measurement products may have components rated with high-voltage isolation barriers, the overall product design, not just the components, dictates whether the device meets high-voltage safety standards. Safety standards have specific requirements for isolating humans from contact with hazardous voltages. These requirements vary among different applications and working voltage levels, but often specify two layers of protection between hazardous voltages and human-accessible circuits or parts.
In addition, the standards for test and measurement equipment are not only concerned with dangerous voltage levels and shock hazards, but also with environmental conditions, accessibility, fire hazards, and valid documentation for explaining the use of equipment in preventing these hazards. They maintain specific construction requirements of isolation equipment to ensure that the integrity of the isolation barrier is maintained with changes in temperature, humidity, aging, and variations in manufacturing processes.
When dealing with safety standards, the European Commission and Underwriters Laboratories, Inc. (UL) have outlined the standards that cover the design of high-voltage instruments. There are approximately 200 individual safety standards harmonized (approved for use to demonstrate compliance) to the Low Voltage Directive, which was the initial document that outlined the specifications for the voltage levels that require safety consideration.
The relevant standard for instrument manufacturers is EN 61010 -- Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use. EN 61010 states that 30 Vrms or 60 VDC are dangerous voltages. In addition to high-voltage design requirements, EN 61010 also includes other safety design constraints (such as flammability and heat). Instrument manufacturers must meet all the specifications in EN 61010 to receive the CE label.
There are two other standards very similar to EN 61010 -- IEC 1010 and UL 3111. IEC 1010, which was established by the International Electrotechnical Commission, is the precursor to EN 61010. The European Commission adopted it and renamed it EN 61010. UL 3111 is also a child of IEC 1010. UL took IEC 1010, made some modifications and adopted it as UL 3111. This new, strict UL standard replaces the older, more lenient UL 1244 standard for measurement, control, and laboratory instruments. For new designs, instrument manufacturers must meet all of the specifications in UL 3111 to receive a UL listing.
Installation Categories
The IEC defined the term Installation Category (sometimes referred to as Over-voltage Category) to address transient voltages. When working with transient voltages, there is a level of damping that applies to each category. This damping reduces the transient voltages (over-voltages) that are present in the system. As you move closer to power outlets and away from high-voltage transmission lines, the amount of damping in the system increases.
The IEC has created four categories to partition circuits with different levels of over-voltage transient conditions.
When configuring a DAQ system, you must take the following steps to ensure that the product meets applicable safety standards:
- Consider the operational environment, which includes the working isolation voltage and installation category.
- Choose the method of isolation in the design based on these operational and safety parameters.
- Choose the type of isolation based on the accuracy needed, the desired frequency range, the working isolation voltage, and the ability of the isolating components to withstand transient voltages.
Not all isolation barriers are suitable for safety isolation. Even though measurement products may have components rated with high-voltage isolation barriers, the overall product design, not just the components, dictates whether the device meets high-voltage safety standards. Safety standards have specific requirements for isolating humans from contact with hazardous voltages. These requirements vary among different applications and working voltage levels, but often specify two layers of protection between hazardous voltages and human-accessible circuits or parts.
In addition, the standards for test and measurement equipment are not only concerned with dangerous voltage levels and shock hazards, but also with environmental conditions, accessibility, fire hazards, and valid documentation for explaining the use of equipment in preventing these hazards. They maintain specific construction requirements of isolation equipment to ensure that the integrity of the isolation barrier is maintained with changes in temperature, humidity, aging, and variations in manufacturing processes.
When dealing with safety standards, the European Commission and Underwriters Laboratories, Inc. (UL) have outlined the standards that cover the design of high-voltage instruments. There are approximately 200 individual safety standards harmonized (approved for use to demonstrate compliance) to the Low Voltage Directive, which was the initial document that outlined the specifications for the voltage levels that require safety consideration.
The relevant standard for instrument manufacturers is EN 61010 -- Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use. EN 61010 states that 30 Vrms or 60 VDC are dangerous voltages. In addition to high-voltage design requirements, EN 61010 also includes other safety design constraints (such as flammability and heat). Instrument manufacturers must meet all the specifications in EN 61010 to receive the CE label.
There are two other standards very similar to EN 61010 -- IEC 1010 and UL 3111. IEC 1010, which was established by the International Electrotechnical Commission, is the precursor to EN 61010. The European Commission adopted it and renamed it EN 61010. UL 3111 is also a child of IEC 1010. UL took IEC 1010, made some modifications and adopted it as UL 3111. This new, strict UL standard replaces the older, more lenient UL 1244 standard for measurement, control, and laboratory instruments. For new designs, instrument manufacturers must meet all of the specifications in UL 3111 to receive a UL listing.
Installation Categories
The IEC defined the term Installation Category (sometimes referred to as Over-voltage Category) to address transient voltages. When working with transient voltages, there is a level of damping that applies to each category. This damping reduces the transient voltages (over-voltages) that are present in the system. As you move closer to power outlets and away from high-voltage transmission lines, the amount of damping in the system increases.
The IEC has created four categories to partition circuits with different levels of over-voltage transient conditions.
- · Installation Category IV - Distribution Level (transmission lines)
- · Installation Category III - Fixed Installation (fuse panels)
- · Installation Category II - Equipment consuming energy from a Category III fixed installation system. (wall outlets)
- · Installation Category I - Equipment for connection to circuits where transient over-voltages are limited to a sufficiently low level by design.
Installation Categories
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