Test leads are connection components designed to work with measurement devices, which means their parameters are adjusted against the functionalities of the meters they are used with. The largest group in terms of quantity contains leads for multimeters (pic. 1).
On the meter side, they end with a banana plug with a normalized diameter of 4mm with a pin — naked or covered in plastic. Covered versions prevent from electric shock from an inspected circuit while connecting and manipulating, as multimeters can be used for measuring high voltages.
Besides multimeters, test leads are used for connecting:
A test lead has to be as flexible as possible, and yet it has to ensure the ability to measure current by the meter (large cross-section) and efficient insulation from high voltage.
This set of criteria is solved by manufacturers with the use of wide assortment, i.e. instead of a single one-for-all lead, they offer highly flexible versions with silicone insulation for high current measurement (cross-section of up to 2.5mm2), or for high voltage measurements.
On the other side, the lead ends with pins, crocodile clips, banana plugs, or hooks, which allow you to mount the end in a point of measurement. With banana-banana leads, you can mount a clip or a hook, which increases flexibility. Wide market offer allows you to choose a lead based on colour and length, with straight or angular plugs.
The applicable IEC/EN61010-1 standard defines four measurement categories for test tools (I-IV).
The higher the category, the better the protection of meters against over-voltage and the quality of the insulation of test leads.
Category numbers are given in conjunction with maximum working voltage that can occur between measurement clamps of a device or between a them and earthing (table 1). Simultaneously, protection against over-voltage is defined.
|Measurement category||Working voltage [V]||Max. over-voltage. [V]||Source impedance [Ω]||Applications|
|II||600||4000||12||Devices in single-phase building networks|
|III||600||6000||2||Industrial devices and lighting in three-phase networks|
|IV||600||8000||Power distribution systems in three-phase networks|
In case of test leads,
the higher the category of a lead, the safer the operator, which is especially important in transient state, where potentials significantly exceed nominal values.
The higher the category, the more covered the test pin in order to extend the distance between the measurement probe and the finger that holds the probe.
Ensuring sufficient protection in case of test leads of higher categories requires special efforts, like using double-insulated leads, as well as covers shutting down the access to metal parts of banana plugs and crocodile clips, preventing operators from touching a metal part.
Because test leads are frequently bent, which results in tears and mechanical damages of the insulation, renowned products feature cables covered with two insulation layers in vibrant colours. When the outer layer is damaged, the colour of the inner layer becomes visible (pic. 3).
Pic. 3. In accordance with the latest requirements of the IEC/EN6101-031 standard, test leads have to have a wear indicator or have to include a cable with double insulation.
This mechanism serves as a warning sign, as it indicates the level of wearing of a lead and informs the user that it should be replaced.
With the latest update of the standard, the IEC/EN6101-031 version obliges manufacturers to implement a wearing indicator in test leads or, should this prove to be impossible, to use leads with double insulation. Additionally, the construction of closed crocodile clips or plugs has to prevent operators from touching metal parts – these are called safe plugs (pic. 4).
Pic. 4. When closed, category II-IV hooks have to have metal parts covered, so that they cannot be touched
the category and current load parameters of a lead have to comply with the functionality of a given device,
as in case of non-compliance, measurement capabilities and safety are determined by the weakest link.
Second group of test leads are cables based on a coaxial cable and BNC plugs on one side, which are mounted to oscilloscopes, generators, frequency meters, and measurement devices that work at high frequencies (e.g. spectrum analyzers (pic. 5).
The other side of such a lead usually ends with mini hooks or the crocodile clip-test pin pair, which allows them to be permanently connected to ground, and to conduct measurements with one hand by touching subsequent points of measurement.
This group also includes cables with BNC plugs on both ends, which are used for connecting laboratory apparatus, BNC-banana cables enabling the use of special probes, and BNC-crocodile cables (naked or covered), commonly used in generators for delivering test signals.
Because some oscilloscopes also work as logical state analyzers, there are specialist solutions available on the market, with BNC plug on one side and goldpin plugs on the other side. This is also one of the applications of BNC-double banana cables, used in electrical measurements, e.g. in power quality analyzers.
For measurements and connections in microwave circuits, we use cables meant for high frequency use, with such plugs as SMA or MMCX. Similarly to leads for meters, coaxial cables are also categorized in terms of maximum working voltage. They are not used in direct methods of current measurements — indirect methods are used instead, such as with the use of clamps or transformers. All of the aforementioned cables come in different lengths, from several dozen of centimetres to ca. 1.5 -2 metres, and usually HF cables are short.
Measuring the parameters of components in SMD packages with the use of regular pins is not an easy thing to do. The components are so small that touching them with measurement pins requires dexterity and using both hands, while the tiniest components may even be destroyed when pressed too hard. Unfortunately, capacitors, inductors or resistors in tiny packages do not have any markings, which means they have to be measured, especially in design offices or service departments.
In such a situation, test tweezers made of plastic, and featuring small metal probes connected with separate cables and ended with covered banana plugs may prove handy (Pic. 6).
It allows for firm and delicate contact of the measuring blades with SMD components using one hand, and ensures appropriate insulation. The other hand can operate the meter.