Test instruments and test leads are a temporary connection to electrical systems, often live at the time, for the purpose of making measurements to establish or confirm various parameters. Andrew Linley, Compliance Director at Electrical Safety UK Ltd looks at some of the standards that exist that we must comply with and that we should take into consideration whilst undertaking measurement tasks using test instruments.
Electrical Test Instruments and Test Leads
Test instruments and measuring devices are used regularly in the electrical installation and maintenance sectors, often without incident, and in many cases without a second thought by the user. We assume that the instruments are capable of undertaking the measurement without risk to the user, and we rarely consider the potential outcomes if things were to go wrong. SI 1989/635, more commonly known as the Electricity at Work Regulations 1989 refers to the need to avoid danger from arising, or protect against its existence.
Regulation 2 defined danger, in the context of electrical systems, as the ‘risk of injury’ and that ‘injury means death or personal injury from electric shock, electric burn, electrical explosion or arcing, or from fire or explosion initiated by electrical energy, where any such death or injury is associated with the generation, provision, transmission, transformation, rectification, conversion, conduction, distribution, control, storage, measurement or use of electrical energy’.
The context of measurement and test instruments can be very wide, this document considers the following instruments used on Low Voltage and Extra-Low Voltage systems for the purpose of testing or measurement:
What does the Law require?
Before looking at the Omnibus Regulation of the Electricity at Work Regulations 1989, it is worth pointing out that, where relevant, all aspects of the Regulations must be complied with, such as:
Regulation 4; systems, work activities, and protective equipment; is where the story commences, with testing typically being carried out to achieve compliance with regulation 4(1) (construction) or regulation 4(2) (maintenance). Regulation 4(3) relates to work activities and requires that safe systems of work be in place such that danger will not arise. This includes testing practices. Many documents have been written to assist in achieving compliance with this, most notably IET Guidance Note 3- Inspection and Testing. HSR25 the Electricity at Work Regulations 1989, Guidance on Regulations makes reference to the preference for all work to undertaken with conductors made dead, with testing included in this requirement- there have to be good reasons (as set out in regulation 14) for live testing to be taking place. Regulation 4(4) relates to equipment provided to be suitable, well maintained, and used properly. It should be noted that since regulation 4(4) is not qualified by ‘so far as is reasonably practicable’, it is absolute in its requirement and compliance is mandatory.
Non-Statutory Guidance
HSG85 Electricity at Work, Safe Working Practices is an HSE publication that considers Actions connected to working dead and working live, including processes for determining which process is most appropriate to allow work to be completed safely, and good practices to be followed to achieve this. All of this is based on Risk Assessment and all works should be planned well in advance.
Testing on energised (or electrically charged) systems or testing on conductors made dead, but in the proximity of parts that are live or electrically charged classes as live working- this includes using an Approved Voltage Indicator (AVI) or test lamp to confirm that a conductor is dead (since until the process is complete the assumption is that the conductor is live and dangerous). Consider that some test instruments (such as an Insulation Resistance Tester) will apply a hazardous voltage to circuits that are dead and again falls into the remit of live working. Where danger might exist, suitable precautions shall be taken.
BS 7671:2018+A1:2020 sets out the requirements for electrical installations, Part 6 details the criteria for inspection and testing, focussing on Initial Verification and Periodic Inspection and Testing. We will be familiar with the documents that are completed and issued being Electrical Installation Certificates, Minor Works Certificates, and Electrical Installation Condition Reports. Part 6 is the UK implementation of IEC 60364-6 Low voltage electrical installations- Part 6- Verification.
Part 6 sets out the criteria for testing and in the case of Initial Verification the sequence in which it should be carried out but does not state how the tests should be completed. IET Guidance Note 3- Inspection and Testing (GN3) is where this information can be found with more condensed guidance found in the IET On-Site Guide.
Section 4 of Guidance Note 3 refers to test instruments and equipment and makes reference to BS EN 61010- Safety requirements for electrical equipment for measurement, control, and laboratory use, and is the basic safety standard for electrical test instruments. Reference is also made to BS EN 61557 Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. Equipment for testing, measuring or monitoring of protective measures. This standard includes performance requirements and requires compliance with BS EN 61010. Guidance Note 3 recognises that leads compliant with HSE document GS 38 should be adequate. Specifically, there is a requirement to protect the user of test instruments when they may be at risk of making contact with circuits that are energised at a hazardous level.
BS EN 50110- Operation of electrical installations, an often overlooked standard, sets out the requirements for functional checks including measurement and testing. A requirement to use suitable and safe instruments by skilled or instructed persons exists and precautions against electric shocks, the effects of short-circuits and arcing must be guarded against. Again, the focus is on working on systems made dead.
GS38 Electrical test equipment for use on low voltage electrical systems is a Health and Safety Executive (HSE) general series guidance note, originally called electrical test equipment for use by electricians. Throughout the document, reference has been made to the reduced risks of systems and therefore testing, carried out on extra-low voltage systems as significantly minimising the dangers associated with electricity, and should be the default approach. Test leads should be appropriately constructed to avoid danger taking account of all of the hazards present, for example, testing on a 24-volt battery may not present an electric shock risk, but short-circuiting battery terminals with excessively long bare test probes carries a short-circuit risk with the potential for heavy current to flow.
Consider a test for continuity carried out with a low resistance ohmmeter, a multifunction test instrument of the continuity setting of a digital multimeter. The circuit should have already been isolated, secured and the absence of voltage confirmed. This being the case, the test instrument will use its internal power source to perform the test, typically at a voltage of between 4 and 24 volts a.c. or d.c. Short-circuiting the test probes will result in indicating a very low resistance measurement, opening the test probes should indicate a very high resistance reading.
On systems that are dead the risk posed is minimal (assuming the check is not being undertaken in a hazardous area such as a potentially explosive atmosphere) and the construction of the test leads need to offer only basic safety features. However, where testing is being carried out within an enclosure containing live parts, or parts that could otherwise be (or could become) charged, more robust test leads must be used and safe systems of work should take into account the additional risks that may be present.
How Accidents Happen
It is recognised that unsuitable test equipment can cause serious burns or electric shock, including from arcing or ‘flashover’. Consideration must also be given to chemical burns (such as from battery acid) or falls from heights as the result of a ‘mishap’.
Common causes of accidents include:
Category of Test Instruments
Not all test instruments are equal, and where on the electrical system we use them is important to note. Some instruments are capable of dealing with the high energy that is present at the origin of an electrical installation, whereas others are typically suited to use on an electrical system where there are varying levels of protection in place between the meter and the supply transformer or other source of supply.
Typically, instruments used at the origin of an electrical installation such as on distribution, and at the point of entry to a building should be CAT IV and rated for the maximum nominal voltage likely to be present at that point. For instruments used within the electrical systems of the building, CAT III may be used, although obviously, instruments rated at CAT IV would also be appropriate. Great care must be taken when using instruments rated at CAT II or lower.
Recommendations for Test Leads and Safe Working Practices
So what do good test leads look like? Here are some features to look for:
To Fuse or not to Fuse?
It is still recognised in the UK that test leads protected by fuses with a maximum rating of 500 mA, offers protection to the user in the event of defective leads or instrumentation in causing rapid disconnection of supply. Care must be taken with regards to test instruments that require a significant current flow to pass, even for a short period of time, such as earth fault loop impedance testers, where a 500 mA fuse will operate at some point in use.
Instruments conforming with the requirements of BS EN 61010 are required to incorporate protection in the instrument, capable of breaking the typical prospective fault current at the intended point of use. Where this is the case, fused test leads are not a requirement but may be used as an additional safety feature.
Modern Approved Voltage Indicators often use a current limiting device as an alternative to fuse protection or in addition to fuse protection.
Electrical Safety First
Electrical Safety First is a campaigning charity that aims to reduce deaths and injuries caused by electricity in the homes. Working with Government, the electrical industry, manufacturers, and retailers they seek electrical safety regulations and standards.
Part of this includes producing a series of Best Practice Guides that can be downloaded from their website, free of charge. Although no document refers specifically to test instrument leads, Best Practice Guide 7 discuss selecting test instruments for given tests and includes test categories with reference to impulse withstand capability.
Further Reading
Electronic test equipment is used to create signals and capture responses from electronic devices under test (DUTs). In this way, the proper operation of the DUT can be proven or faults in the device can be traced. Use of electronic test equipment is essential to any serious work on electronics systems.
Practical electronics engineering and assembly requires the use of many different kinds of electronic test equipment ranging from the very simple and inexpensive (such as a test light consisting of just a light bulb and a test lead) to extremely complex and sophisticated such as automatic test equipment (ATE). ATE often includes many of these instruments in real and simulated forms.
Generally, more advanced test gear is necessary when developing circuits and systems than is needed when doing production testing or when troubleshooting existing production units in the field.[citation needed]
Types of test equipment
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Basic equipment
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Keysight commercial digital voltmeter checking a prototypeThe following items are used for basic measurement of voltages, currents, and components in the circuit under test.
The following are used for stimulus of the circuit under test:
Voltcraft M-3850 portable multimeterThe following analyze the response of the circuit under test:
And connecting it all together:
Advanced or less commonly used equipment
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Meters
Probes
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A multimeter with a built in clamp facility. Pushing the large button at the bottom opens the lower jaw of the clamp, allowing the clamp to be placed around a conductor (wire). Depending on sensor, some can measure both AC and DC current.Analyzers
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Signal-generating devices
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Leader Instruments LSG-15 signal generator. Cable testerMiscellaneous devices
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Platforms
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Keithley Instruments Series 4200 CVUSeveral modular electronic instrumentation platforms are currently in common use for configuring automated electronic test and measurement systems. These systems are widely employed for incoming inspection, quality assurance, and production testing of electronic devices and subassemblies. Industry-standard communication interfaces link signal sources with measurement instruments in “rack-and-stack” or chassis-/mainframe-based systems, often under the control of a custom software application running on an external PC.
The General Purpose Interface Bus (GPIB) is an IEEE-488 (a standard created by the Institute of Electrical and Electronics Engineers) standard parallel interface used for attaching sensors and programmable instruments to a computer. GPIB is a digital 8-bit parallel communications interface capable of achieving data transfers of more than 8 Mbytes/s. It allows daisy-chaining up to 14 instruments to a system controller using a 24-pin connector. It is one of the most common I/O interfaces present in instruments and is designed specifically for instrument control applications. The IEEE-488 specifications standardized this bus and defined its electrical, mechanical, and functional specifications, while also defining its basic software communication rules. GPIB works best for applications in industrial settings that require a rugged connection for instrument control.
The original GPIB standard was developed in the late 1960s by Hewlett-Packard to connect and control the programmable instruments the company manufactured. The introduction of digital controllers and programmable test equipment created a need for a standard, high-speed interface for communication between instruments and controllers from various vendors. In 1975, the IEEE published ANSI/IEEE Standard 488–1975, IEEE Standard Digital Interface for Programmable Instrumentation, which contained the electrical, mechanical, and functional specifications of an interfacing system. This standard was subsequently revised in 1978 (IEEE-488.1) and 1990 (IEEE-488.2). The IEEE 488.2 specification includes the Standard Commands for Programmable Instrumentation (SCPI), which define specific commands that each instrument class must obey. SCPI ensures compatibility and configurability among these instruments.
The IEEE-488 bus has long been popular because it is simple to use and takes advantage of a large selection of programmable instruments and stimuli. Large systems, however, have the following limitations:
LAN eXtensions for Instrumentation
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The LXI (LXI) Standard defines the communication protocols for instrumentation and data acquisition systems using Ethernet. These systems are based on small, modular instruments, using low-cost, open-standard LAN (Ethernet). LXI-compliant instruments offer the size and integration advantages of modular instruments without the cost and form factor constraints of card-cage architectures. Through the use of Ethernet communications, the LXI Standard allows for flexible packaging, high-speed I/O, and standardized use of LAN connectivity in a broad range of commercial, industrial, aerospace, and military applications. Every LXI-compliant instrument includes an Interchangeable Virtual Instrument (IVI) driver to simplify communication with non-LXI instruments, so LXI-compliant devices can communicate with devices that are not themselves LXI compliant (i.e., instruments that employ GPIB, VXI, PXI, etc.). This simplifies building and operating hybrid configurations of instruments.
LXI instruments sometimes employ scripting using embedded test script processors for configuring test and measurement applications. Script-based instruments provide architectural flexibility, improved performance, and lower cost for many applications. Scripting enhances the benefits of LXI instruments, and LXI offers features that both enable and enhance scripting. Although the current LXI standards for instrumentation do not require that instruments be programmable or implement scripting, several features in the LXI specification anticipate programmable instruments and provide useful functionality that enhances scripting's capabilities on LXI-compliant instruments.[3]
VME eXtensions for Instrumentation
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VME eXtensions for Instrumentation (VXI) are an electrical and mechanical standard used mainly with automatic test equipment (ATE). VXI allows equipment from different vendors to work together in a common control and packaging environment. VPX (a.k.a. VITA 46) is an ANSI standard based on the VMEbus with support for switched fabric using a high speed connector. VXI combines VMEbus specifications with features from the general-purpose interface bus (GPIB) to meet the needs of instrumentation applications. Other technologies for VME, VPX and VXI controllers and processors may also be available.
Selecting VME, VPX and VXI bus interfaces and adapters requires an analysis of available technologies. The original VME bus (VMEbus) uses Eurocards, rugged circuit boards that provide a 96-pin plug instead of an edge connector for durability. VME64 is an expanded version of the VMEbus that provides 64-bit data transfers and addressing. VME64 features include asynchronous data transfers, an addressing range between 16 and 40 bits, data path widths between 8 and 64 bits, and a bandwidth of 80 Mbit/s. VME64 extended (VME64x) is an improved version of the original VMEbus that features a 160-pin connector family, 3.3 V power supply pins, bandwidths up to 160 Mbit/s, injector/ejector locking handles, and hot swap capability. VME160 transfers data at 160 Mbit/s. VME320 transfers data at a rate of 320 Mbit/s. VXI combines VMEbus specifications with features from the general-purpose interface bus (GPIB) to meet the needs of instrumentation applications. VME, VPX and VXI bus interfaces and adapters for VPX applications are also available.[4]
PCI eXtensions for Instrumentation
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PCI eXtensions for Instrumentation, (PXI), is a peripheral bus specialized for data acquisition and real-time control systems. Introduced in 1997, PXI uses the CompactPCI 3U and 6U form factors and adds trigger lines, a local bus, and other functions suited for measurement applications. PXI hardware and software specifications are developed and maintained by the PXI Systems Alliance.[5] More than 50 manufacturers around the world produce PXI hardware.[6]
Universal Serial Bus
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The Universal Serial Bus (USB) connects peripheral devices, such as keyboards and mice, to PCs. The USB is a Plug and Play bus that can handle up to 127 devices on one port, and has a theoretical maximum throughput of 480 Mbit/s (high-speed USB defined by the USB 2.0 specification). Because USB ports are standard features of PCs, they are a natural evolution of conventional serial port technology. However, it is not widely used in building industrial test and measurement systems for several reasons (e.g., USB cables are rarely industrial grade, are noise sensitive, are not positively attached and so are rather easily detachable, and the maximum distance between the controller and device is limited to a few meters). Like some other connections, USB is primarily used for applications in a laboratory setting that do not require a rugged bus connection.
RS-232 is a specification for serial communication that is popular in analytical and scientific instruments, as well for controlling peripherals such as printers. Unlike GPIB, with the RS-232 interface, it is possible to connect and control only one device at a time. RS-232 is also a relatively slow interface with typical data rates of less than 20 kB/s. RS-232 is best suited for laboratory applications compatible with a slower, less rugged connection.
Test script processors and a channel expansion bus
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One of the most recently developed test system platforms employs instrumentation equipped with onboard test script processors combined with a high-speed bus. In this approach, one “master” instrument runs a test script (a small program) that controls the operation of the various “slave” instruments in the test system, to which it is linked via a high-speed LAN-based trigger synchronization and inter-unit communication bus. Scripting is writing programs in a scripting language to coordinate a sequence of actions.
This approach is optimized for small message transfers that are characteristic of test and measurement applications. With very little network overhead and a 100 Mbit/s data rate, it is significantly faster than GPIB and 100BaseT Ethernet in real applications.
The advantage of this platform is that all connected instruments behave as one tightly integrated multi-channel system, so users can scale their test system to fit their required channel counts cost-effectively. A system configured on this type of platform can stand alone as a complete measurement and automation solution, with the master unit controlling sourcing, measuring, pass/fail decisions, test sequence flow control, binning, and the component handler or prober. Support for dedicated trigger lines means that synchronous operations between multiple instruments equipped with onboard Test Script Processors that are linked by this high-speed bus can be achieved without the need for additional trigger connections.[7]
Test equipment switching
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The addition of a high-speed switching system to a test system's configuration allows for faster, more cost-effective testing of multiple devices, and is designed to reduce both test errors and costs. Designing a test system's switching configuration requires an understanding of the signals to be switched and the tests to be performed, as well as the switching hardware form factors available.
See also
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References
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