Residual Current Devices (RCDs) Explained

Residual Current Devices (RCDs) Explained

Table of Contents

  1. Electric Shock
  2. Nomenclature
  3. Working Principle
  4. Connection
  5. Types
  6. Relationship Between Grounding Systems
  7. Limitations

Electric Shock

Electric shocks occur when a person accidentally touches a live electrical component whether directly or indirectly.

Given the current levels considered dangerous, a maximum permissible value of 30 mA is considered safe. For LV systems, the dominant component in body impedance is skin resistance, which depends essentially on the environment (dry, humid, or wet conditions)

IEC has defined the "conventional touch voltage limit", noted UL, as the maximum touch voltage that can be maintained indefinitely under the specified environmental conditions. The value used is 50 V AC rms. This value is consistent with an average impedance value of 1700 ? and a maximum current of 30 mA.

However, grounding systems like TN provide a safe path for current to flow in case of a fault.

But what happens when the grounding system fails? For example, the connection between the equipment is cut, This is when RCDs come in place.

Charles Dalziel (1904-1986) from the USA. Saved countless lives when he came up with the brilliant idea of GFCI in 1960


Fig 1: Charles Dalziel

Nomenclature

To not get lost with all these abbreviations let's break it down first.

  • RCD: Residual Current Device. Generic term covering devices incorporating residual current protection so in fact all the coming types are types of RCDs in other words the term "RCD" is often used as an umbrella term for all devices that protect against residual current.
  • RCCB: Residual Current Circuit Breaker. Without overload or short circuit protection.
  • RCB: Residual Circuit Breaker: It is the same as RCCB
  • RCBO: Residual Current Breaker with Over-Current Protection
  • ELCB: Earth Leakage Circuit Breaker. ELCB is the old name and often refers to voltage-operated devices that are no longer available and it is advised you replace them if you find one. ELCBs have the following disadvantages. Voltage ELCBs cannot detect leakage from the phase wire to other earthed bodies and only trip when current flows through the earth conductor. It cannot detect low leakage current due to its lower sensitivity. The current ELCB is renamed as RCCB or RCD while the voltage ELCBs are still known as ELCB but they are obsolete.
  • ELR: Earth Leakage Relays. They are used in combination with dedicated external current transformers to measure and detect Residual currents. However, since the relay's job is only to monitor, there is always a requirement for a remote switching element to interrupt the circuit upon alarm conditions such as electromagnetic contactors or motorized circuit breakers
  • GFCI: Ground Fault Circuit Interrupters. GFCI and RCD are essentially two names for the same device. In the UK and Europe, it is called an RCD. In the US it is called a GFCI. Now there are differences between the US and Europe. In the US the triggering current is 5 mA. In Europe, it is 30 mA. In the US they are electronic while in Europe they typically are electro-mechanical. In the US they often are in sockets while in, Europe they are more commonly in the panel.

Why Multiple Names?

The variation in names arises due to:

  • Regional Differences: Different countries or regions adopt different standards and terminology.
  • Industry Preferences: Different industries may prefer certain terms based on historical use or specific applications.
  • Technical Distinctions: While the basic function is the same (detecting residual currents), some terms like RCCB specifically indicate a device without overcurrent protection, while RCD is more general and can refer to devices with or without overcurrent protection.


Working Principle

The working part of an RCD consists of an iron core, called a toroid, one half of which is wrapped by the live copper wire while the other half is wrapped by the neutral copper wire.

Using the physical properties of electromagnetism, the neutral and live cables produce equal and opposing magnetic fields in the toroid which cancel each other out.

A special switch, called a relay is connected to the toroid via a third copper wire, sometimes referred to as the 'search' or 'detector' coil.

During normal operation, no electric current passes through this wire to the relay, because the live and neutral coils are not producing a net electromagnetic field.

During Earth Leakage, The current in the neutral cable is now greater than that in the live, so the magnetic fields they produce are no longer equal, resulting in a net magnetic field in the toroid. This magnetic field creates a current that passes down the detector coil to the relay, causing it to snap open, breaking the circuit and thus cutting power to the electrical system.

Check Figures 2 and 3 for a clearer view


Fig 2: Circuit of RCD


Fig 3: Construction of RCD

RCDs are provided with a test function: The test button connects a resistor between the live terminal on the load side, to the neutral on the supply side of the RCD. Pushing the test button creates an imbalance across the trip coil, resulting in a current flow in the trip coil and operation of the trip relay and opening of the RCD contacts.

When you press the test button, the resistor allows a small amount of current to bypass the neutral conductor, creating a difference (or imbalance) between the live and neutral currents. This imbalance mimics the effect of a real fault (such as a leakage current due to insulation failure or a person accidentally touching a live part).

The integrity of an RCD should be tested regularly. All sockets and fixed RCD should be tested about every one to three months

Check this video for more about RCD testing.


Connection

RCDs must have the neutral wire, and this is a crucial difference between it and MCB or MCCB as the latter has the 2-pole as an option for extra protection in the single-phase system. While in RCDs it is a must to have the neutral pass through its terminal. A typical connection for single and three-phase RCDs is shown in Figure

?

Fig 4: Three-phase System Connection
Fig 5: Single-phase System Connection

RCDs Types

RCDs can be categorized by:

Design

  1. Fixed (Din Rail Mounted): Installed at a fixed location in the electrical distribution board or consumer unit. Protects a specific circuit or a group of circuits. Typically used in permanent electrical installations in residential, commercial, and industrial settings. Offers comprehensive protection against residual currents for a designated area. They have a mark or a ‘Test’ pushbutton on the front panel, indicating it is a fixed RCD.
  2. Socket-Outlet (Power Point): Installed in individual socket outlets or incorporated into extension leads. Provides localized protection for specific appliances or devices plugged into the protected socket. Convenient for retrofitting and adding an extra layer of protection to specific outlets. Commonly used in areas where portable electrical equipment is connected.
  3. Portable Plug-in: Designed for temporary or portable use and can be plugged into existing outlets. Often referred to as plug-in RCDs or portable residual current devices. Suitable for use with various electrical appliances and tools, providing on-the-go protection. Frequently used in outdoor settings, construction sites, and for DIY projects. Offers flexibility as it can be moved and used with different power sources.

Fig 6: Designs of RCDs

Implementation

RCD function can be implemented in the form of:

  1. Dedicated devices like RCCB
  2. Modules attached to the MCCB to convert it to RCBO
  3. Electronic function within the microprocessor of an ACB

Sensitivity

  1. High-sensitivity, tripping on 10mA or 30mA residual current. Used in homes and residential
  2. Medium sensitivity, 100mA, 300mA, or 500m
  3. Low sensitivity, 500mA. Used in industrial enterprises

Based on the Electrical Waves

When selecting for RCD, the type of electrical wave to be monitored is very important to be taken into consideration. Otherwise, improper or even no interruption could happen when residual current is exceeded.


Fig 7: RCDs Types According to Electrical Waves

  1. Type AC: Designed for general-purpose use
  2. Type A: Designed for equipment incorporating electronic components
  3. Type F: For equipment with frequency-controlled speed drives
  4. Type B: A specialist RCD for particular three-phase applications including electric vehicle chargers and solar photovoltaic systems


Relationship Between RCDs and Ground Systems

Grounding system type is very essential for the RCD connection. Below is a reminder of different earthing systems.


Fig 8: Earthing Systems

For the mentioned systems:

  • TN-S: RCD use is optional to fulfill the additional protection against electric shock. Since there is dedicated protection provided by the protective earth conductor
  • TN-C-S: RCD use is optional but conditional to installation only downstream after the separation of PE and N conductors.
  • TN-C: RCD use is not allowed. Because there will be no difference even during the earth leakage. Check the following figure.

Fig 9: Example of Using RCD in TN-C System (RCD Fails to Detect the Leakage)

  • TT: Use of RCD is very important as main network isolator. This is because in such systems the contribution of the ground fault current is very low. Secondary use of RCD nearby electrical loads is also preferable. Certain conditions of the network must be carefully checked for proper RCD operation such as earth resistance of electrical equipment.
  • IT: Since there is no return path the ground fault won't represent a problem to human lives However, the use of RCD is preferable. However, the location of RCD installation is very important for a proper operation. in IT networks, RCD should be placed as near as possible to the electrical load which will make it more possible for a second earth fault to occur on a point before the RCD enabling it to work properly.


Setting

Current:?In the UK standard domestic RCDs operate at 30mA. In other words, they will allow a current imbalance below this level in order to account for real-world situations and avoid 'nuisance tripping', but will cut power as soon as they detect a current leakage of 30mA or above.

Speed:?UK regulation BS EN 61008 stipulates that RCDs must trip within certain time frames depending on the amount of current imbalance.

  • 1 x In = 300ms
  • 2 x In = 150ms
  • 5 x In = 40ms

'In' is the symbol given to tripping current - so, for example, 2 x In of 30mA = 60mA.RCDs used in commercial and industrial environments have higher mA ratings of 100mA, 300mA and 500mA.


Limitations

  • RCD will not protect against live-neutral shocks, because the current in the live and neutral is balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals of a light fitting), you may still get a nasty shock
  • RCDs are known not to detect overload conditions on phase-to-phase short circuits and phase-to-neutral short circuits
  • RCD will not protect against the overheating that results when conductors are not properly screwed into their terminals

Thanks for reading.

Gollapalli Sreenivasulu

M.tech ,M.I.E, C Eng(E) S.M.I.I.E, M.I.E.T,M(IEEE) ,NSC

3 个月

Insightful!

Christopher Horne

2023 eFIXX Lecturer of the year, Riverside College, Widnes

3 个月

要查看或添加评论,请登录

社区洞察

其他会员也浏览了