In electrical products the flow of current is conducted through wires coated in insulation. This insulation blocks the current and keeps it in the electrical circuit, thus protecting the user. However, all components leak some current and most forms of insulation are not 100% resistant to current. Small amounts of current leakage are safe and OK for the user, but where an insulating material reaches dielectric breakdown, this is where the current leakage becomes dangerous.
Dielectric breakdown is the point at which excessive voltage is applied to a component so that the insulating material becomes a conducting material, ie. instead of the insulating material trapping current safely inside the conductor, the insulating material itself becomes a conductor.
If you consider a common conducting material such as copper wire, this contains free electrons which can move around easily with little resistance, while an insulating material such as rubber or plastic has no free electrons. It therefore takes a high amount of voltage to move these electrons out of their stable state. This high voltage is known as the breakdown strength of the material, ie. the point at which a physical transformation of the conductive values takes place and the disruption of the current flow within the circuit can cause an electric shock in the user. This is usually seen via arcing (the electric current visibly jumps in the air and can cause fire) or through direct contact with the current.
So dielectric testing comes in two forms:
- Dielectric breakdown testing – submitting a component to excessive voltage to establish at what point insulating material fails or breaks down.
- Dielectric withstand test – submitting a component to normal voltages and monitoring the amount of leakage that occurs, to check that insulating material of the component can withstand voltage variations under these normal conditions.
How do we carry out dielectric testing?
With an everyday electrical component such as a laptop charger or phone charger the high voltage end of the circuit is what we plug into the wall socket, and the low voltage end is what we plug into the electrical item, ie. the laptop or phone. So to carry out dielectric testing we attach a test lead to the high voltage end of the cable and a second lead to the low voltage end.
We can then send electrical current through the component at different levels to see what current leakage occurs. We can take a note of the results as different voltages are sent through. If the component doesn’t leak current above certain levels, as stated in applicable standards, when certain levels of voltage are applied in the dielectric withstand test, then we can state that the component is safe. And if no dielectric breakdown occurs at excessive voltage then we know the insulating material is safe and sufficient and resistant to this particular hazard.
When would the dielectric withstand test be used?
The dielectric withstand test would be carried out by the manufacturer of an electrical component at the end of the production process. It is a finished product testing process in this case. However, even under normal operating conditions a component can fail because it has been subjected to dirt, humidity or vibration. These conditions can cause current leakage and therefore create a shock hazard and short circuiting, so it is important to test a finished product, but also for electrical engineers to periodically test an electrical component in a working environment. For this they would use the dielectric withstand test.
Please note that this section is for information purposes only. Anyone using equipment referred to in this section must be suitably qualified and/or experienced within the respective field. If in doubt before use, please consult a qualified electrician or engineer & thoroughly read all instruction booklets.