A residual current protective device (RCD), commonly known as a differential switch (or referred to as a differential switch), is a safety device that protects against electric shock - both in the event of direct and indirect contact. It limits the effects of damage to receivers connected to the installation. It is used as an additional protection in addition to automatic power cut-off that operates in the event of a direct phase-housing short circuit. Thanks to it, in the event of a short circuit in electrical installations, there is a smaller chance that equipment will be damaged or electrocuted.
How is a differential constructed and what are its types?
The components of an RCD are:
- contacts of current paths , together with the lock and lever switching on the device,
- differential release , most often it is a polarized relay,
- a circuit breaker testing device that allows the switch to be checked during use,
- Ferranti transformer – a ferromagnetic ring through which the phase wires and neutral wire pass.
The last element is crucial, it is responsible for measuring leakage current. A differential circuit breaker works by switching off an electrical circuit, e.g. a lighting circuit, if the geometric sum of the currents flowing in and out of it is not zero. A differential (leakage) current then appears.
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There are several ways to divide differentials. The first one concerns the operating current (sensitivity) – differential circuit breakers differ from each other in the differential current they support:
- up to 30 mA – highly sensitive devices, used where the risk of electric shock and damage to the installation is high, e.g. in bathrooms, kitchens, workshops or greenhouses,
- 30–500 mA – medium-sensitive switches installed in residential buildings, offices or on construction sites,
- above 500 mA – low-sensitivity switches, used in circuits with high leakage current, and also as the main switch of the electrical installation.
RCDs are additionally distinguished by detecting differential current : type AC – sinusoidal alternating current, A – sinusoidal alternating current, half-wave rectified sinusoidal current and pulsed current, B – as A and additionally direct current. The next two categories are RCCB – without built-in overcurrent protection and RCBO with such protection.
Most often, highly sensitive switches (up to 30 mA) are used , which are activated at a differential current of 20 mA. Lower current is relatively safe – a person shocked by electric current is able to break away from the damaged installation.
Standards and regulations for residual current circuit breakers
The Regulation of the Minister of Infrastructure of 12 April 2002 on the technical conditions that buildings and their location should meet requires the use of RCDs in paragraph 183, section 1, which states that residual current protective devices should be used as a supplement to basic protection against electric shock and fire. Subsequent regulations are brought by standards. PN-IEC 60364-5-53:1999 states that an RCD without a protective conductor is not an effective means of protection against indirect contact. The PN-HD 60364-4-41: 2009 standard imposes the obligation to install highly sensitive differential devices in plug-in sockets with a rated current of up to 20 A and outdoor mobile devices (up to 32 A). An important supplement is the provision in PN-HD 60364-4-41:2017-09 requiring the use of differential devices in lighting circuits in households. The previous one, PN-HD 60364-4-41:2009, was limited to bathroom lighting.
RCDs should also be installed in temporary lighting installations , such as fairs, circuses, outdoor events (PN-HD 60364-7-740:2009), exhibitions with lighting fixtures (PN-HD 60364-5-559:2010) and bathrooms (PN-HD 60364-7-701:2010)
Polish Standards, unless they are cited in an act or regulation, are not mandatory. However, installers should comply with them, because they protect them in the event of a dispute or accident related to the electrical installation performed . This is confirmed by the Construction Law, which states that persons performing independent technical functions in construction are responsible for their performance in accordance with the regulations and principles of technical knowledge, and they are also to ensure the diligence of the work, its organization, safety and quality.
Homeowners and business owners are not always aware of how important it is to take care of high-quality residual current circuit breakers. It is worth making customers aware that their installation can protect against serious problems - not only inconvenience, but also electric shock or damage to equipment.
The lighting circuit is one of the elements of the home electrical network. Others include, for example, the plug socket circuit and the circuit designed for devices with a power of 2 kW and more. The need to use differential circuit breakers does not result solely from standards. In home installations, they are important devices that protect household members from electric shock when touched directly or indirectly. In addition, they limit the effects of damage to devices, including the risk of fire, in the event of a failure affecting the electrical installation.
It is also important to use highly sensitive residual current devices. The maximum operating current of 30 mA results from the fact that at higher values, in the event of an electric shock, respiratory tract paralysis may occur. The devices are able to operate above 15 mA. They usually will not react below this threshold, due to the certainty of the power supply. Then, the power supply may be interrupted due to natural leakages in the installation. However, in special situations, RCDs with a differential current of even 10 mA are used.
Circuit breakers are used primarily in TN-S and TN-CS network systems. The first of these is a grounded system with separated protective and neutral conductors (PE and N). This solution is most often used in houses and apartments. TN-CS is grounded, normally non-conductive parts are connected to the neutral point of the transformer, partially has a PEN conductor, further divided into PE and N.
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When not to use RCDs
Differential devices are not used in TN-C systems found in old buildings . This is prohibited by the PN-IEC 60354-5-53:1999 standard, which states that it cannot be considered an effective means of protection against indirect contact, even if the rated differential operating current does not exceed 30 mA. In this system, the PEN conductor is both the working and protective conductor, and the RCD disconnects the phase and neutral conductors simultaneously, so it is also protective. It will also not work when voltage appears on the lamp housing - for the transformer there will be no difference between the incoming and outgoing current and it will not work. On the other hand, if the housing is not well insulated from the ground, it can disconnect the circuit due to a small current leakage through it to the grounded elements. According to the applicable regulations, TN-C installations in private houses and apartments must be replaced with TN-S or TN-CS.
Differentials are also not installed in emergency lighting circuits, due to the need for their continuous operation . Such systems are out of reach, and their operation is handled by specialists with a SEP qualification certificate.
You will find an attractive offer of RCD devices in the Onninen online wholesaler
If you are planning to install a residual current circuit breaker, check out the selected products at the Onninen electrical wholesaler :
The Schneider Electric switch is often used in both industrial and general construction, protecting against electric shock and reducing the risk of fire. (In accordance with national standards and IEC 60364) Its advantages include not only high quality and excellent parameters, but also time saving - there is no need to dismantle the entire connecting rail - and greater comfort of work (thanks to padlocks).
Eaton double-pole circuit breaker with a rated short-circuit breaking capacity of 6kA. It offers, among other things, the possibility of busbar connection from the top and bottom, connection of the power supply from the top and bottom and connection to 48 VDC on the run. It has safety certificates CLS(new)1 and CLS(new)2. The exact technical parameters can be found on the dedicated product page.
Its main advantages include electrical durability up to 10,000 operations and mechanical durability up to 20,000. It can be used in flush-mounted installations. It has a declaration of conformity HN_Pl.
Legrand switch with energy limitation class 3, suitable for flush-mounted installations, with a mechanical durability of up to 20,000 operations. Designed for mounting on TH35 rail.
This is a high-quality Volt Polska power supply device, providing protection in the event of a power outage. It is used both to ensure continuous operation and protection against damage in heating installations, as well as for computers, servers and power tools.
Our offer includes high-quality circuit breakers – 6kA, 10kA, 15kA and 25kA. Diverse parameters and attributes make it easy to choose exactly the device you need.