The next question is – What is a FINAL CIRCUIT Or not exceeding 32 amps for a circuit supplying fixed equipment only.
With a rated current not exceeding 63 amps for a circuit that has one or more sockets This tells us that we should use the disconnection times in TABLE 41.1 on page 59 for FINAL CIRCUITS
On page 58 you will find Regulation 411.3.2.2 Let’s start on page 58 of the 18th Edition Wiring Regulations book to determine the disconnection times and then we can look at pages 61 onwards to find the maximum Zs values. So now that we know why we have disconnection times, where do we get the information from? The lower the Zs of the circuit, the greater the fault current that flows and the quicker the disconnect times. And if our circuit meets the requirements for Zs, or loop impedance as it is called, then we should be confident that it will meet the required disconnection times. Once we know what the disconnection time should be for safety, according to the regulations, then we can use the tables in the Wiring Regulations to determine what the maximum Zs or loop impedance is for different circuits. The quicker that a circuit can disconnect the supply during a fault, the safer the circuit is. The bigger the voltage, the bigger the danger of injury or death, and as you will see, as the voltage increases, the time to disconnect becomes more critical. The disconnection time, the time taken for a circuit breaker to cut off the electrical supply during a fault is a big factor in someone surviving an electrical shock. We will be using information and numbers from the Wiring Regulations, BS7671 18th Edition. It is all about ensuring that an electrical shock is survivable by an average, healthy adult. The permitted time to disconnect from the electrical source and to make safe the circuit is very much dependent upon the voltage of the circuit and a few other factors. If a high touch voltage is suspected between two metallic points in close proximity, it can be measured using a meter with an AC voltage measurement option.We are sometimes asked why the Wiring Regulations have disconnection times for electrical circuits.Īnd what is the purpose of knowing how quickly a fuse or circuit breaker should disconnect an electrical circuit during a fault. This is a very good test to use for spot checks on touch voltage as you can just wander around an installation, touching a probe briefly on the metal surfaces to look for hazardous voltages.įigure 2 – Checking for touch voltage on earthed surfaces A touch contact enables the user to simply touch their finger on the contact while a single test lead is applied to the metalwork under question with the meter warning of any significant voltage, as shown in figure 2. If a high touch voltage is suspected between a metallic surface and the ground, it can be checked using a meter with a touch voltage contact, such as the MFT1700 series. If the test current is expected to produce a dangerous voltage, the tester should inhibit the test and show a warning, such as >50 V. This will apply to earth loop impedance testing and RCD testing since these tests involve producing a current on the earth. Test instruments meeting the requirements of BS EN 61557 must perform a pre-check when testing to ensure that the earth potential does not rise to a dangerous level during the test. Some special locations, such as swimming pools, require that the touch voltage does not exceed 25 V. Requirements for touch voltageĪccording to section 411.5.3 of BS7671:2008 Amendment 3, touch voltage must not exceed 50 V at any location on an installation. This could be the taps in a kitchen or bathroom, metallic light switches, radiators or the casing of an appliance. Touch voltage is tested to ensure that people cannot get an electric shock from simply touching a metallic surface or two metallic surfaces simultaneously. On other types of earthing systems, an earth resistance this high would indicate a fault.įigure 1 - Hazardous touch voltages between different earthed surfaces Why test for touch voltage?
Section 411.5.3 of BS7671:2008 Amendment 3 notes that RA can be as high as 200 Ω. It is common to see a significant touch voltage on TT earth systems where the earth resistance (RA) can be quite high. The touch voltage (V) results from the current flow to earth (I Δn) multiplied by the resistance of the earth (R A), so that the touch voltage is found by: The voltage exists if there is a higher resistance earth path to the exposed metalwork than to the ground beneath your feet or another piece of exposed metalwork in close proximity, as shown in figures 1a & 1b. The term touch voltage refers to the shock hazard present on any exposed metalwork or protective conductor.