RCD Protection (PDF)

RCD protection & devices Eng. Albert Zerafa Eng. Albert Zerafa 2024 -2025 1 Earth Leakage Circuit Breakers. – Residual current device Eng. Albert Zerafa 2024 -2025 2 What is an RCD? RCD – Re...

RCD protection & devices Eng. Albert Zerafa Eng. Albert Zerafa 2024 -2025 1 Earth Leakage Circuit Breakers. – Residual current device Eng. Albert Zerafa 2024 -2025 2 What is an RCD? RCD – Residual Current Device RCCB – Residual current circuit breaker A device that is designed to provide protection against shock or electrical fires by cutting off the flow of electricity automatically when it senses a ‘leakage’ of electric current from a circuit. To appreciate the importance of an RCD it is helpful to understand how much electrical energy it takes to kill a human being. The smallest fuse used in a normal electric plug is 3 Amps; it takes less than one twentieth of that current to kill an adult in less than one tenth of a second. RCDs offer a level of personal protection that ordinary fuses and circuit breakers cannot provide Eng. Albert Zerafa 2024 -2025 3  Different equipment for different protection Remember ! MCBs are designed to protect circuits and work on current overload. Remember! RCDs are intended to protect people and work on circuit imbalance. Eng. Albert Zerafa 2024 -2025 4  RCD Protection RCDs are typically designed to operate within 40ms and to disconnect the electricity supply when they sense harmful leakage, typically 30 milliamps. The sensitivity and speed of disconnection are such that any earth leakage will be detected and automatically switched off before it can cause injury or damage. Analysis of electrical accidents show the greatest risk of electric shock results from contact between live parts and earth. As a general rule, an RCD will prevent a person from being subjected to a lethal shock from a fault current to earth by limiting the magnitude of the shock and the duration of the shock. An RCD will give no protection from a live to neutral contact. Every RCD unit is fitted with a test button which should be operated regularly to prove breaker operation. Eng. Albert Zerafa 2024 -2025 5  Requirements for RCDs ref GN5 p53 RCDs or RCBOs are required: Where earth fault loop impedance is too high to provide the required disconnection time. E.g where the distributor does not provide an earth. In this instance the use of an RCD or RCBO is for fault protection not for additional protection. For socket outlets with a rating not exceeding 32A. For circuits within locations containing a bath or shower. For mobile equipment with a current rating not exceeding 32A for use outdoors For cables without earth metal covering In many special installations and locations – including exhibitions, fairgrounds, floor heating, fountains, pools, caravans, boats, electric vehicle charging Local installation regulations Eng. Albert Zerafa 2024 -2025 6  Low voltage Isolation, control and Protection Systems. An RCD will significantly reduce the risk of electric shock, however, an RCD will not protect against all instances of electric shock. If a person comes into contact with both the Live and Neutral conductors while handling faulty plugs or appliances causing electric current to flow through the person's body, this contact will not be detected by the RCD unless there is also a current flow to earth. Eng. Albert Zerafa 2024 -2025 7  Low voltage Isolation, control and Protection Systems. Electric Shock commonly falls into two categories – shock resulting from either: Direct Contact with the electrical supply Indirect Contact via exposed conductive parts or metalwork that have become live due to a fault. In a healthy electrical system, insulation between the conductors, neutral connections and earth is sound. Unfortunately, this is not always the case, therefore there are various safety devices incorporated within electrical systems that attempt to ensure electrical safety either to the appliances, conductors or users. A correctly installed earthing system should minimise the effect of electrical shock. Eng. Albert Zerafa 2024 -2025 8  Effects of Electric Shock on the Human Body What is an electric shock: A dangerous physiological effect resulting from the passing of an electric current through the human body. The value of the current passing through the human body depends on the body impedance. The value of body impedance can vary enormously according to circumstances. Initial current flow can be quite low but will start to increase rapidly as even small currents will quickly burn through the surface of the skin resulting in significant drop in the external impedance. The effect of electric current passing through the human body become more severe as the current increases. Eng. Albert Zerafa 2024 -2025 9  Electric Shock Barely perceptible, Throw off, painful Muscular contraction, no harmful effects sensation can’t let go Ventricular fibrillation and death Impaired breathing Eng. Albert Zerafa 2024 -2025 10 Effect of different values of electric current flowing through the human body ( at 50 Hz) Eng. Albert Zerafa 2024 -2025 11 Eng. Albert Zerafa 2024 -2025 12 Eng. Albert Zerafa 2024 -2025 13 Eng. Albert Zerafa 2024 -2025 14 Eng. Albert Zerafa 2024 -2025 15 How does the RCD work? Eng. Albert Zerafa 2024 -2025 16  How does the RCD work In the previous analogy, the leakage is detected without measuring the leak itself. It is the flow and return rates that are measured and compared. Similarly, in an RCD, when the line and neutral currents are equal, the RCD will not trip but when it senses that the neutral current is less than the line current, it will trip. An RCD compares the line and neutral currents and switches off the electricity supply when they are no longer equal. Eng. Albert Zerafa 2024 -2025 17 Basic Principle of Operation of an RCD – cont’d Eng. Albert Zerafa 2024 -2025 18  Basic Principle of operation of the RCD When the load is connected to the supply through the RCD, the line and neutral conductors are connected through primary windings on a toroidal transformer. In this arrangement, the secondary winding is used as a sensing coil and is electrically connected to a sensitive relay, the operation of which triggers the tripping mechanism. When the line and neutral currents are balanced, as in a healthy circuit, they produce equal and opposite magnetic fluxes in the transformer core with the result that there is no current generated in the sensing coil. (For this reason the transformer is also known as a ‘core balance transformer’). When the line and neutral currents are not balanced, they create an out-of-balance flux. This will induce a current in the secondary winding which is used to operate the tripping mechanism. It is important to note that both the line and neutral conductors pass through the toroid. Eng. Albert Zerafa 2024 -2025 19 Eng. Albert Zerafa 2024 -2025 20 Test Circuit of RCD RCDs work equally well on single phase, three phase or three phase and neutral circuits, but when the neutral is distributed it is essential that it passes through the toroid. The test circuit is designed to pass a current in excess of the tripping current of the RCD to simulate an out-of-balance condition. Operation of the test button verifies that the RCD is operational. It is important to note, therefore, that the test circuit does not check the circuit protective conductor or the condition of the earth electrode. All RCDs should be checked at regular intervals to confirm that the RCD trips. As a minimum, a check every six months is recommended. Eng. Albert Zerafa 2024 -2025 21  Risk of Electrocution 1. It only takes 40 mA or more to cause irreversible damage to a normal cardiac cycle (ventricular fibrillation) or death. 2. When someone gets in direct contact with 230 V mains voltage and earth, the current flowing through the body is just around 230 mA. 3. To protect against serious injury and death we need to disconnect the supply in 40 ms at 230 mA. 4. For lower values of shock current, longer disconnection times may be acceptable, but if disconnection takes place within 40 ms fibrillation is unlikely to occur. Eng. Albert Zerafa 2024 -2025 22  Risk of Electrocution 1. ‘High sensitivity’ RCD’s rated at 30mA, installed to provide personal protection, are designed to disconnect the supply within 40 ms at 150 mA (x 5) and within 200 ms at rated tripping (x 1) current to protect the user. 2. ‘Medium sensitivity’ devices, rated 100 mA or more will provide protection against fire risks but will NOT provide full personal protection. 3. A fuse or circuit breaker alone will not provide protection against electric shock. 4. Even with a 30 mA RCD fitted, a person coming into contact with mains voltage will suffer an electric shock. Eng. Albert Zerafa 2024 -2025 23  RCD Considerations The rated operating current of the RCD is not the only consideration; the speed of tripping is also very important, if ventricular fibrillation is to be avoided. The RCD should be used as additional protection only and not as a substitute for the basic means of direct contact shock protection. Eng. Albert Zerafa 2024 -2025 24 RCD Selection – Protection Categories Protection Categories Personal Protection – Where the minimum operating current of the RCD is not greater than 30 mA and RCD operates to disconnect the circuit within the specified time. Installation Protection – Devices used to protect against the risk of fire caused by electrical faults. RCDs operating at residual current levels up to and including 300 mA. Eng. Albert Zerafa 2024 -2025 25  RCD Selection - Classifications RCD Classifications General operation – these operate instantaneously and do not have an intentional time delay in operation and thus cannot discriminate. Time delayed operation – these provide selectivity in circuits where RCDs are connected in series. (e.g Type S) RCDs with built in time delays should not be used to provide personal protection Eng. Albert Zerafa 2024 -2025 26 Eng. Albert Zerafa 2024 -2025 27 Cascading RCDs General RCD’s operate instantaneously and when two or more RCDs are installed in series, more than one device may trip. Time delayed RCDs provide selectivity where RCDs are connected in series. It is essential to install devices which incorporate a time delay, upstream of the general device, so that the device nearest the fault will trip. Selectivity is achieved between RCDs when: the upstream RCD is of selective type (type S or time-delayed type with appropriate time delay setting), and the ratio of the rated residual operating current of the upstream RCD to that of the downstream RCD is at least 3:1. It is not possible to achieve selectivity with two S type RCDs in series. Eng. Albert Zerafa 2024 -2025 28 Eng. Albert Zerafa 2024 -2025 29 Developments in RCDs Eng. Albert Zerafa 2024 -2025 30  Different Types of RCD BS7671 18th Edition, Section 531.3 – Residual Current Devices (RCDs): Regulation 531.3.3 (Types of RCD) ‘Different types of RCDs exist depending on their behaviour in the presence of DC components and frequencies The appropriate RCD shall be selected from: Type AC Type A Type F Type B Some RCDs offer adjustment of sensitivity. These RCDs should not be accessible to unauthorised persons. For the majority of cases, a type AC devices are suitable with type A and type B being used where special circumstances exist. Eng. Albert Zerafa 2024 -2025 31 Symbols to indicate the different types of RCD Type AC, Type A, Type F and Type B RCDs are not suitable for use in DC supply systems. Eng. Albert Zerafa 2024 -2025 32  RCD selection ref: IEE 18 th ed ‘n p154 RCD Examples of type of Equipment / load Type AC Resistive, Capacitive, Inductive loads generally without any electronic components, typically: Immersion heater/Oven/Hob with resistive heating elements Electric shower Tungsten and halogen lighting Type A Single phase with electronic components, typically: Single phase invertors Class 1 IT and multimedia equipment Power supplies for Class 2 equipment Appliances such as a washing machine that is not frequency controlled e.g. d.c or universal motor Lighting controls such as dimmer switch and home and building electronic systems LED drivers Induction hobs Electric Vehicle charging where any smooth DC fault current is less than 6 mA Type A is also suitable for Type AC applications Eng. Albert Zerafa 2024 -2025 33  RCD selection ref: IEE 18 th ed ‘n p154 RCD Examples of type of Equipment / load Type F Frequency Controlled equipment / appliances, typically: Some washing machines, dishwashers and driers e.g. containing synchronous motors Some class 1 power tools Some air conditioning controllers using variable frequency speed drives Type F is also suitable for the Type AC and A applications Type B Three phase electronic equipment typically: Inverters for speed control UPS Electric Vehicle charging where any smooth DC fault current is greater than 6mA Photovoltaic Power Electronic Converter Systems (PECS) typically: Industrial machines Cranes Type B is also suitable for Type AC, Type A and Type F applications Type B+ Type B+ RCDs are not recognised in BS7671 and do not have an international or harmonised (BS EN) standard Eng. Albert Zerafa 2024 -2025 34 Operation and maintenance Testing by end user: – All RCDs should be tested at least once every six months to ensure that they are still operative. – This involves operating the test device (normally a push button) marks ‘T’ or ‘Test’. – This should cause the RCD to trip, disconnecting the supply to the protected circuit. Reinstate the supply by reclosing the device or pressing the ‘Reset’ button as appropriate. – If the RCD does not switch off when the test button is pressed, the user should seek expert advice. Eng. Albert Zerafa 2024 -2025 35