Molly Ayala
Voltage spikes, also known as power surges, can cause significant damage to electrical equipment and electronics. They occur due to various reasons, including voltage fluctuations from large motor drives or lightning strikes. To protect against these spikes, it is essential to test electrical lines for their ability to withstand sudden surges in current. This is known as spike testing or surge testing. The test involves simulating a spike or surge event by subjecting the electrical line to a higher voltage than its nominal input, ensuring that the cable can handle the fault current for a minimum duration. This process helps identify potential vulnerabilities and ensures the reliability of the system. While there are cheap and convenient methods to test for spikes, it is important to follow health and safety procedures to ensure that the cable is dead and safe before conducting any tests.
| Characteristics | Values |
|---|---|
| Purpose | To prove that a cable can withstand a full specified fault current for a minimum period of 3 seconds |
| Testing procedure | The cable's outer sheath is removed, the screen unravelled and 'pigtailed', and any semi-conducting inner layers and tapes are removed. The insulation is then partially stripped, and a lug is placed and crimped at the conductor end. The phase connector is connected to a single-phase 50Hz power supply, and a spike is hammered into the centre of the test cable sample. |
| Equipment | A signal generator, a BAUR test set, a power meter, a cheap spike meter (ac-powered piezo alarm with the switch turned on but the wire broken and spliced with a 1mm air gap), a line filter |
| Results | Performance criteria are graded A, B, or C. A means no change in the system; B means the system auto-recovers; C means the user must intervene. |
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What You'll Learn
Use a cheap spike meter to test for line transients
To test for line transients, you can use a cheap spike meter, also known as a digital multimeter. Voltage transients are just a technical term for voltage spikes.
It is important to note that you do not need an expensive spike meter unless you are a professional electrician. A cheap spike meter can be an AC-powered piezo alarm with the switch turned on but the wire broken and spliced with an exact 1mm air gap and sealed with tape. This will yield a 1kV arc gap of +/-25% 1kV/mm for a smooth gap. So when line transients occur > 1kV, you will know you need a more expensive line filter.
When using a multimeter, it is important to ensure that it is suitable for the voltage you are testing. Each multimeter category is based on the amount of power available, and the voltage level and category rating are the two major factors in choosing the correct meter. Do not use a meter/probe rated less than the voltage of the equipment being tested. Additionally, make sure the meter is in good working condition, as faulty parts or components can cause arc flash.
To capture voltage transients on an oscilloscope, you will need a scope with memory capabilities and a probe that can withstand the transient voltage without breaking down. Set the triggering on the scope for fast AC on the channel you are using and the time base to around 1ms per division. Set the vertical input to a suitable high level, taking into account the attenuation of the probe. It will probably take several tries with the trigger set to different levels before you capture the spike.
There are also other methods and tools to detect spikes, such as using a Hall effect current sensor, an oscilloscope with a Pearson coil or a shunt resistor, or LEM sensors.
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Prepare the cable by removing the outer sheath
To prepare a cable for spike testing, the outer sheath must be removed. This is a necessary step to ensure the cable can withstand a sudden surge in current and voltage. The outer sheath provides mechanical protection and a moisture barrier for low, medium, and high-voltage cables.
There are specialised tools available for removing the outer sheath, such as the Alroc PG range of cable sheath and jacket strippers, which are pliers that deliver a circular and longitudinal cut to the cable outer sheath. The Alroc OGA heavy-duty cable outer sheath removal tool is another option, which enables a longitudinal cut to be made on the outer sheath of single and multi-core cables. These tools are designed to extend the working life of the cable jointer by providing four different cutting thickness positions and two cutting edges on the same blade.
When removing the outer sheath, it is important to be cautious. In some cases, it may not be legal or safe to remove the outer sheath, especially when the cable is inside a conduit. The outer sheath provides essential protection to the wires, and without it, the insulation on the individual conductors may not be sufficient.
Some screened cables include a ripcord under the outer sheath, which facilitates the removal process. This ripcord allows for the correct tearing of the outer sheath when pulled, ensuring the inner screen remains undamaged.
Once the outer sheath is removed, the screen can be unravelled and 'pig-tailed' to maximise connectivity and conductivity.
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Simulate a live cable installation
To simulate a live cable installation, you can use IPS Cable Simulation, a software that provides real-time capabilities for motion analysis of flexible cables. This simulation software helps engineers optimise the routing, length, and shape of flexible cables and hoses. It also allows for interactive optimisation of routings and lengths, as well as the ability to customise clips and create a clip database.
Using IPS Cable Simulation, you can identify risk factors such as tensions, small bending radius, and points of collision. Additionally, you can optimise parameters like material properties and cable lengths in real time. The software provides validated results regarding flexible cables and related parts early in the product process, reducing the need for prototyping and iterations, saving time and money.
Another aspect of simulating a live cable installation is testing for voltage spikes. Spike testing for cables ensures that the cable can withstand a sudden surge in current and the maximum designated voltage. This involves preparing the cable by removing the outer sheath, unravelling and 'pig-tailing' the screen to maximise connectivity and conductivity. Once the cable is prepared, it is connected to a power supply and a spike is hammered into the centre of the test cable sample.
For a basic understanding of voltage spikes, a cheap spike meter can be used. However, for more precise measurements, a line filter is recommended.
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Identify the cable with a signal generator
To identify a cable with a signal generator, you can use a cable tracer tester, also known as a toner tracer tester. This device is used to test the continuity of cables and identify specific pairs of wires within a multi-pair cable. It can also be used to locate breaks and faulty cables.
- Prepare the cable by removing the outer sheath and unravelling and 'pig-tailing' the screen to ensure maximum connectivity and conductivity.
- Remove any semi-conducting inner layers or tapes and partially strip the insulation to reveal the conductor.
- Connect the phase connector of the cable to a single-phase 50Hz power supply.
- Connect the wire screen to the earth.
- Using the signal generator, send a 25 MHz sine wave down the cable.
- Measure the attenuation at the other end of the cable.
- Use the visual signal indicator and speaker on the signal generator to locate the cable.
- For accurate identification of data cable pairing, use the multiple tone pattern feature on the signal generator.
It is important to note that this process may vary depending on the specific signal generator and cable being used. It is always recommended to refer to the manufacturer's instructions and take appropriate safety precautions when working with electrical equipment.
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Use a line filter for protection against spikes
A line filter is a device that can be used to protect against electrical spikes. Line filters are also known as surge protectors or suppressors, and they are placed between an electrical device and an external line.
Line filters are designed to reduce electromagnetic interference (EMI) in AC mains. They usually consist of an inductor, capacitor, and resistor, and they work to minimise ripples in output voltage. Some line filters also offer minimal voltage regulation, while others protect against power-quality issues.
Line filters are particularly useful for protecting against spikes caused by lightning, EMP, switching spikes, and EMI/RFI generated by inverters. They can be used in conjunction with TVS devices to provide additional protection.
When choosing a line filter, it is important to consider the specific requirements of your electrical system. For example, if you have a solar backup system, a hybrid inverter may already include a line filter. Additionally, if you live in an area with frequent lightning, it is recommended to use a line filter with a high voltage protection rating.
While line filters can provide effective protection against spikes, they may not be suitable for all electrical systems. In some cases, a low pass line filter may be more appropriate for isolating certain parts of an electrical system rather than protecting the entire system. It is always recommended to consult with a qualified electrician to determine the best protection methods for your specific electrical setup.
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Frequently asked questions
A cable spike test is designed to prove that a cable with a wire screen can withstand a full specified fault current for a minimum period of 3 seconds. It simulates the accidental "spiking" of a live cable installation and ensures that the circuit protection will operate and the circuit fails safely.
You can test for spikes in your electrical line with a spike meter. A cheap spike meter can be made with an ac-powered piezo alarm with the switch turned on but the wire broken and spliced with a 1mm air gap. When line transients occur above 1kV, you will know you need a line filter.
A spike meter is a device that can detect voltage spikes in your electrical line.
Surge testing is the process of subjecting an electrical system to voltage spikes to ensure the resilience and reliability of the system. It helps to identify and mitigate potential vulnerabilities or weaknesses that could lead to failures during voltage spikes or power surges.
