High Voltage Measurement and Isolation

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There are many issues to consider when measuring high voltage. When specifying a data acquisition (DAQ) system, the first question you should ask yourself is whether or not the system will be safe. Making high voltage measurements can be hazardous to your equipment, to the unit under test, and even to you and your colleagues. To ensure that your system is safe, you should provide an insulation barrier between the user and hazardous voltages with isolated measurement devices.

For in-depth guidance on making a voltage measurement, visit the how-to guide.

Table of Contents

  1. What is Isolation?
  2. Isolation Considerations
  3. Isolation Types
  4. Isolation Topologies
  5. Safety and Environmental Standards
  6. Typical Applications Requiring Isolation
  7. National Instruments Isolated Products

What is Isolation?

There are many issues to consider when measuring high voltage. When specifying a data acquisition (DAQ) system, the first question you should ask yourself is whether or not the system will be safe. Making high voltage measurements can be hazardous to your equipment, to the unit under test, and even to you and your colleagues. To ensure that your system is safe, you should provide an insulation barrier between the user and hazardous voltages with isolated measurement devices.

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 DAQ system, and level shift the signal ground reference to a single system ground. Safety isolation references standards that have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high-voltage and transient voltages to be transmitted across its boundary to other electrical systems that the user may come in contact with.

Incorporating isolation into a data acquisition 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 your 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 60Hz 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 1 below). This potential is generally not a DC level; thus, the result is a noisy measurement system often showing power-line frequency (60 Hz) components in the readings.


Figure 1. A Grounded Signal Source Measured with a
Ground-Referenced System Introduces Ground Loop


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, thus preventing any current from flowing between multiple ground points.

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 may 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 will completely reject, rather than measure, 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 common-mode rejection ratio (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 2. In this circuit, CMRR in dB is measured as 20 log Vcm/Vout where V- = Vcm.


Figure 2. CMRR Measurement Circuit



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.

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Isolation Considerations


There are several terms to be familiar with 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 the product is subjected to during testing to ensure conformance.

Transient Voltage (Overvoltage): A brief electrical pulse or spike that may 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, above the transient voltage. A device cannot operate safely near this voltage for an extended period of time.

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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.


Figure 4. Optocoupler


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.


Figure 5. 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.


Figure 6. Capacitor
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Isolation Topologies

It is important to understand the isolation topology of a device when configuring a measurement system. Different topologies have several associated cost and speed considerations.

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.


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 all of the channels can be accessed in parallel. A more cost-effective but slower architecture involves multiplexing each isolated input channel into a single ADC.

Another method of providing channel-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.















Figure 7. Channel-to-Channel Multiplexed Topology


Bank
Another isolation topology involves banking, or grouping, several channels together to share a single isolation amplifier. 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. Individual channels are not isolated, but banks of channels are isolated from other banks and from ground. This topology is a lower-cost isolation solution because this design shares a single isolation amplifier and power supply.


Figure 8. Bank Topology
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Safety and Environmental Standards

When configuring a data acquisition system, you must take several steps to ensure that the product meets applicable safety standards. First, consider the operational environment. This includes the working isolation voltage and installation category. Next, 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 or not 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. 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 Overvoltage 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 overvoltages are limited to a sufficiently low level by design.


Figure 9. Installation Categories
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Typical Applications Requiring Isolation

Single-phase AC Monitoring
To measure power consumption with 120V / 240V AC power measurements, you record instantaneous voltage and current values. Your final measurement, however, may not be instantaneous power, but average power over a period of time or cost information for the energy consumed.
By making voltage and current measurements, software can make power measurements or do other analyses. To make high-voltage measurements you will need some type of voltage attenuator to adjust the range of the signal to the input range of the measurement device. Current measurements require a precision resistor. The voltage drop across the resistor is measured, and Ohm’s Law (I = V/R) produces a current value.

Fuel Cell Measurement
Fuel cell test systems make a variety of measurements that require signal conditioning before the raw signal can be digitized by your data acquisition system. An important feature for the testing of fuel cell stacks is isolation. Each individual cell may generate about 1 V, and a stack of cells may produce several kV. To accurately measure the voltage of a single 1V cell in a large fuel cell stack requires a large common-mode range and high common-mode rejection ratio. Because adjacent cells have a similar common-mode voltage, bank isolation is sometimes acceptable.


High Common Mode Thermocouple Measurement
Some thermocouple measurements involve high common mode voltages. Typical applications include measuring temperature while a thermocouple is attached to a motor, or measuring the temperature dissipation capabilities in a conductive coil. In these cases, you are trying to measure small, mili-volt changes with several volts of common mode voltage. It is therefore important to use an isolated measurement system with good common-mode rejection specifications.

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National Instruments Isolated Products


National Instruments offers many products that provide isolation for your measurement and automation application. Offerings include SCXI, SCC and Fieldpoint product lines, as well as high-voltage relays, switches, and DMMs. Most of the isolation products that National Instruments makes fall under the standards outlined by IEC 1010-1 and UL 3111-1, which address standards for measurement, control, and laboratory use.


SCXI for High Voltage Measurements and Isolation

Signal Conditioning eXtensions for Instrumentation (SCXI) is a signal conditioning and data acquisition system for PC-based instrumentation applications. An SCXI system consists of a shielded chassis that houses a combination of signal conditioning input and output modules, which perform a variety of signal conditioning functions. You can connect many different types of sensors and signals directly to SCXI modules. The SCXI system operates as a front-end signal conditioning system for PC plug-in or PCMCIA data acquisition boards.


Figure 10. SCXI Signal Conditioning and Data Acquisition System



The SCXI-1125 is an 8 channel isolated analog input module. The SCXI-1125 is CE certified as double insulated, Category II, providing 300 Vrms of working isolation between channels and channels-to-ground. The module also features programmable gains and low-pass filtering on each analog input channel, and the input range can be expanded to 1,000 VDC with the TBX-1316 terminal block. This architecture is ideal for amplification and isolation of millivolt sources, volt sources, 0 to 20 mA and thermocouples.

The SCXI-1121 and SCXI-1122 are designed for a wide variety of sensor and signal inputs requiring isolation. The SCXI-1121 offers independently configurable isolation amplifiers, filters, and excitation sources for each channel. The SCXI-1122 is a multiplexed module with a single isolation amplifier, filter, and excitation source for all channels. Both modules offer 250 Vrms working isolation and support strain, RTD, thermocouple, millivolt, volt and 0 to 20 mA current input signals.

SCC Isolated Products

National Instruments SCC is a portable, modular signal conditioning system for use with E Series and low-cost data acquisition (DAQ) devices. SCC products condition a variety of analog input and digital I/O signals. With this modular design, you choose your conditioning on a per-channel basis.


Figure 11. SCC Signal Conditioning


The NI SCC-A10 is a dual-channel module that accepts input voltage sources up to 100 V. Each channel of the SCC-A10 includes a 10x attenuation circuit and differential instrumentation amplifier with low-impedance outputs for maximum scanning rates by the DAQ device. The attenuation circuit includes high-impedance bias resistors, so you can connect floating or ground-referenced inputs to the NI SCC-A10 without adding external bias resistors. The SCC-A10 also provides overvoltage protection (up to 250 Vrms) for your DAQ system. SCC-AI analog input modules provide 300V of Category II isolation. Each module provides low-pass filtering and input ranges from ±50 mV to ±42V.


Fieldpoint for High Voltage Measurements

National Instruments FieldPoint and Compact FieldPoint are modular, embedded control and distributed I/O systems for measurement, control, and data logging applications that demand industrial-grade hardware with easy installation and configuration. Both Compact FieldPoint and FieldPoint feature built-in signal conditioning for direct connectivity to sensors and actuators. Modules are available for connecting to thermocouples, RTDs, strain gauges, 4-20 mA signals, high voltage sources, and many other signals. The FieldPoint products support both embedded control by running LabVIEW Real-Time on a dedicated embedded processor and connectivity to a PC via a variety of industrial buses (Ethernet, serial, CAN, Foundation Fieldbus). The FieldPoint products are designed to operate in harsh environments with electromagnetic noise, wide temperature ranges, and high shock and vibration.

The FieldPoint product lines can measure and switch high voltage signals. All the modules are isolated from the backplane with 250V working isolation and 2300V transient over voltage protection. The [c]FP-AI-102 is a 12 bit analog input module capable of measuring +/- 120V DC. The [c]FP-DI-330 is a digital input module for up to 250V AC or DC, and the [c]FP-RLY-420 and FP-RLY-422 are relay modules capable of switching up to 250VAC and 120 VDC.









Figure 13. FieldPoint Industrial Control and Measurement



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