Differential switches are compulsory in all domestic electrical installations. They serve to protect people from the risk of death from “direct contact” with the live parts of the installation (cables), or from “indirect contact” with the metal casings of electrical appliances accidentally subjected to voltage due to a fault in the insulation of their installation, and to protect against fires that could be caused by these bridge connections due to insulation failure.
Differential switches primarily protect people and then the installations.
In recent years, major changes have come about in terms of the electrical appliances and other equipment used in homes. In particular, we have begun to use different electronic components such as electronic reactances for lighting, electronic light intensity variators, electronic igniters, power supplies and frequency variators (installed in computers, energy-saving lamps, air conditioners, TVs, music players, etc.) which, along with storms, often cause differential switches to trip suddenly.
To adapt to these new receivers and to avoid as far as possible any sudden trips, differential switches have evolved to improve their operation.
- Types of differential switches for domestic use
AC-type residual current devices The symbol is 
This type of differential switch is sensitive only to alternating leakage currents. They were the first on the market and should be present in any domestic installation. They work correctly in installations that have practically no electronic components and are located in zones where storms are infrequent. However, if the house is located in a zone where there are frequent summer storms and atmospheric discharges (lightning) that cause power surges and earth leakages, or if its interior installation has several receptors with electronic components (smart electrical appliances, low-consumption or dimmable lighting, computer power supplies, air conditioning units, music player, television, etc.) where there could be pulsed direct current leakages (which can be as dangerous as alternate current but is not detected by AC residual current devices), residual current devices of this type are not suitable for installations with electronic components or in areas where there are frequent storms causing them to suddenly trip due to the distortion caused by these receptors. In fact, they are no longer permitted in some countries, like Germany and Switzerland.
Differential class A. Their symbol is 
This type of differential switch is sensitive to alternating and continuous leakage currents and also includes a high-frequency filter which improves its magnetic core to prevent unexpected trips caused by harmonics generated when there are different receptors with electronic components. These features mean this type of differential switch is suitable for installations with receivers with electronic components.
Superimmunised A-type differential switches. Their symbol is 
or 
This type of differential switch is the same as the A-type, but its trip curve approaches the limits set by Standard 61008-1, enabling it to function correctly in installations with strong loads of receptors with electronic components, such as electronic lighting reactances, variable speed drives, electronic igniters and where there is a large number of computers.
This type of differential switch also includes an energy accumulation circuit which significantly reduces sudden trips caused by occasional earth leakages associated with transient surges caused primarily by atmospheric discharges, differentiating them from a real defect. This means that this type of differential switch is sensitive to alternating and continuous leakage currents, greatly reducing sudden trips and making them more suitable for installations with receivers with electronic components or ones located in areas with frequent storms.
There are other types of differential switches, such as “Delayed” (S-type) for installations upstream of other differential switches; “Industrial” (B- and F-type) for recharging vehicles, engine ignitions and variable speed drives; and “Rearmable” for unattended installations that require a guarantee of continuity of service, although they are more generally used in industrial and commercial installations. But in domestic supplies, to maximise protection of people and prevent sudden trips, the conclusion is that:
For domestic use, it is recommended to always install superimmunised differential switches
- Installation recommendations
In accordance with the ITC-BT-25 Guide of the Electrotechnical Regulations for LV (low voltage) in domestic installations, the following instructions are recommended to be followed:
To prevent a single differential switch from disconnecting certain devices such as computer equipment, refrigerators and freezers. For this type of circuit it is highly advisable to always provide the installation with individual differential protection. Where there is a single differential switch or several in parallel, they will have an instantaneous trip (0.05s) and a maximum sensitivity (differential-residual intensity) of 30 mA. But if there are differential switches installed in series, there must be selectivity between them. This is achieved if the differential switch located upstream has a certain delay in its activity (S-type) and if its sensitivity is at least three times greater than that of differential switches located downstream. Never install the differential switches upstream of the surge protections. At least one differential switch shall be fitted for every five installed circuits. For the electrification of a terrace, garden or exterior installation, at least one differential switch independent of the interior circuits must be installed. In the event of frequent unexpected tripping and after checking that this is not due to insulation faults or differential misalignment:*Separate the kitchen and oven or air conditioning circuit from the rest.
*Replace the differential switch that trips for a superimmunised switch, and if it is already a superimmunised switch, replace it with another one that is “Rearmable”. Avoid installing excessively long cables downstream of a differential switch in order to reduce the intensity of the earth leakage currents. - Recommendations for ensuring the safety of persons
- Ensure a good grounding, guaranteeing the continuity of the current in the event of a defect, both for the protection lines (green/yellow earth cable) and the electrodes (earth bars).
- Maintain proper insulation, both for the different devices that are connected to our interior installation and for the cabling and fuse boxes it is composed of, as they establish the capacity and insulation of the installation.
- Ensure a good connection between the protective conductor and the ground of the receiver to avoid dangerous voltages for people who may come into contact with the cases of the different receivers and devices.
- Ensure continuity of the ground loop between the protective earth circuit and the metallic masses of the different receivers or apparatus.
- Press the “Test” button of each differential switch once a month to ensure it is working properly.
