By Pat Shaw, CEO, Major Tech

Pat Shaw, CEO, Major Tech

When dealing with electrical systems and components, safety is more than simply making sure the instrument you use has the relevant standards mark. Many a time when an electrician or engineer intends measuring an installation, there could be instances where they are measuring the unknown. The question the user should be asking is: “If I make a mistake, will my meter protect me”.

The IEC (International Electrotechnical Commission) has over the years, introduced safety standards for various electrical components including Test and Measuring Instruments. The IEC has prepared an International and European safety standard – the IEC 61010-1 – which defines the safety requirements for measuring instruments.

In particular, the IEC 61010-1 standard defines also the measurement areas called ‘categories’, shortly indicated with the abbreviation ‘CAT’. These categories are CAT I; CAT II; CAT III; and CAT IV – and the most dangerous one is CAT IV. Exceeding or deviating from application parameters can lead to inaccurate measurements or injury.

A CAT rating mark is easy to put on a device – but it is what is inside the device that verifies the validity of that mark. If you look inside the device you can see precisely how the printed circuit board (PCB) is constructed, what components are used, what protection is used and if it can withstand electrical faults.

Over-voltage spikes

When using a measuring instrument on an electrical system, a fault might result in a spike of electricity (over-voltage spikes) that can be dangerous to the technician doing the measurements.

In a case like this, you want to be sure your instrument has been designed to absorb the spike and protect the technician. Similarly, if a technician makes a mistake while on working with an instrument, you want to know that a potential spike resulting from that mistake will stop at the instrument and not in the person.

At the very least, injury to a person can result in delayed maintenance and repairs and, at worst, can result in costly medical bills, occupational health expenses as well as the legal ramifications that could result from such injury. Of course, damage to the installation can also be costly, but less so than the harm caused to an individual.

The ultimate goal of any rating system, specifically the IEC 61010-1 standard, is to ensure technicians make use of the right tool for the job, thereby ensuring the safety of the person using it and the safety of the system as a whole.

Global measurement standards

The key to safety when buying electrical instruments is to ensure the device has been designed for the particular job at hand. The most reliable way to do this is by making sure the instrument chosen has the required rating from the International Electrotechnical Commission (IEC). As noted, the standards in question are the IEC 61010-1 standards which are aimed at safety in electrical measuring instruments.

The IEC 61010-1 standard defines safety measurements in various categories, ranging from CAT I to CAT IV for voltages up to 1 000 V. The higher the category, the closer to the main electricity supply the system being measured is and the more dangerous it is if there is a fault.

Voltage transients are defined as short duration surges of electrical energy and are the result of the sudden increase of energy previously stored or induced by other means, such as heavy inductive loads or lightning.

The categories and where they can safely be used are:


Generally, the measurement category CAT IV is for measurements in an area at the origin or near the origin of installation. The area includes:

  • An incoming service cable whether it is an underground cable or an overhead line to a detached building.
  • The outside and service entrance.
  • The service drop, from a pole to a building.
  • The tails from the power electricity meter to the main protective device (breaker or fuse) or main distribution board.
  • The secondary side of medium voltage power transformer and the main distribution board close to it.

N.B. Instruments designed for CAT IV may also be used in all lower category ratings III, II and I and when an even higher degree of reliability and availability is desired.


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Generally, the measurement category CAT III is for measurements performed to an area inside the building installation. The area includes:

  • Equipment in fixed installations such as main switchgear and distribution boards.
  • Stationary motors with permanent connections, Busbars, junction boxes and sub main cables in industrial plants.
  • Short branch circuits, fuse or circuit-breaker panels and some mains installation control equipment.
  • Fix lighting systems and wiring including cables.
  • Three-phase and single-phase appliance sockets.


Generally, the measurement category, CAT II is for measurements performed on circuits directly connected to a low voltage installation. Examples:

  • Portable (for example, drills, hair dryer, table lamp, etc) or fixed (for example, fridge, water heater, etc) or equipment powered via a socket outlet.
  • Household appliances, portable tools and similar equipment.


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Generally, the measurement category, CAT I is for measurements performed to the following examples:

  • Equipment intended to be connected to a mains supply in which means have been taken to substantially and reliably reduce overvoltage spikes to a level where they cannot be a hazard.
  • Any high-voltage (but with low-energy source) derived from a high-winding resistance transformer, such as the high-voltage section of a photocopier.
  • Signal level circuits for telecommunications and electronic equipment.

For example, an instrument rated CAT II 300 V (this is equivalent to a CAT I 600 V rating) indicates that the instrument can withstand an impulse voltage of 2 500 V, but it should not be used to measure mains CAT II circuits operated at over 300 V. The instrument is not suitable for measurements on CAT III or CAT IV circuits.

Make the right decision

When purchasing a voltage testing instrument, or any measurement device, people often simply approach an electrical wholesaler and ask for a device. Unfortunately, they mostly ask for the cheapest instrument that will suit their purpose and don’t specify an instrument with a certain category rating.

A manufacturer can claim to ‘design to’ a standard but may not offer independent verification so, while generally adhering to the safety ratings, opting for the cheapest device is never the best option. Many cheaper devices on the market may seem to be capable of doing the job, but when one looks at the components making up the instrument, they are of poor quality and the design is sub-par. That means when an over-voltage occurs, the device will not be able to protect the user.

The buyer may be able to save 10% or 20% on the cost of the device, but they take the risk of personal harm if something goes wrong. Similarly, the wholesalers could find themselves in legal trouble for selling an instrument rated for certain jobs, but which cannot offer the required protection.

As mentioned, the closer you measure to the main supply, the higher the category required to ensure the safety of the user because of the design requirements and components involved. This means that a CAT IV 600 V instrument will cost slightly more than a CAT III 600 V device, because you’re paying for your own safety.

In summary

Understanding the dangers and the reason for category ratings means that purchasing an instrument is not simply a case of looking for the cheapest option available. It is vitally important to consider how the device is designed, manufactured and the components used. The brand you select is also important and you must be confident that the company takes the necessary care in designing and manufacturing the instrument, as well as supporting it with technical know-how.

Major Tech has a 30-year long history of providing a broad range of quality electrical test instruments manufactured to the highest standards, making Major Tech a brand technicians can trust to deliver the safety and service, as well as the functionality promised.

More info:           +27 (0)11 822 1551



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