TNC-S Outsourcing Supplies – Think Outside the Box | NAPIT


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Paul Chaffers, Technical Events Manager and Technical Author for NAPIT’s Site Solutions Guide, takes a closer look at the potential issues associated with the use of TNC-S supplies for outbuildings. The question that contractors often ask is: “Can I use a TN-CS (PME) grounding system to equip an outbuilding?” Unfortunately, the answer is not always black and white, and like a lot of areas of electrical installation design, it requires engineering judgment to be applied. This article is a small excerpt from NAPIT’s on-site solutions guide, which contains many installation examples and explores various possibilities while providing useful solutions for everyday electrical installation tasks. Multiple Protective Grounding (PME) limitations When supplying an outhouse from a TN-CS (PME) supplier, we need to be aware of the problems that could occur. BS 7671 prohibits the use of PME in some locations; Caravan gardens and moorings have special requirements, for example. This is because the Electrical Safety, Quality, and Continuity Regulations (ESQCR) prohibit connecting a PME grounding facility to any metalwork in a recreational residence vehicle. Further requirements regarding PME apply to most sections of Part 7 under BS 7671 for private installations or locations. but why? PME is a TN-CS system, in which the neutral and ground functions are combined into a single conductor (PEN conductor) on the supply side for installation. The PEN conductor is denoted to earth in multiple positions using the ground electrodes, as shown in Figure 1. For overhead supply cables, this is implemented in the transformer and many utility poles between the transformer and the mount. Underground cables are usually grounded along the length of the range with electrodes. Historically, PME systems have the advantage of being supported by numerous cross connections to earth via metal tubes, etc., which are connected to the grid via protective links to nearby installations, thus providing some form of additional grounding. This accidental connection should never be relied upon, but it does play a huge role in PME grounding systems. While the neutral ground is then separated inside the fixture, PME properties can cause problems when PME is used outside the fixture; One issue is known as a perceived shock. Figure 1 shows the perceived shock that the main building is being supplied via the TN-C distribution system, with the nearest ground pole somewhere outside in the street. This distance can be great, maybe 30 or 40 meters. The perceived shock can arise from touching any metallic object connected to the PME system of the main building, such as an item of Class 1 equipment, during real contact with the ground. For outbuildings, like our example, there could be a potential difference between real land and land exported from the main building. This is due to the voltage drop in the PEN conductor, due to the returning load current, which is something that occurs under normal operating conditions. The problem is made worse when the body’s resistance is low. For example, imagine having a garden party in the summer. All the kids jump in and out of the pool, saturating the grass, and I set up a makeshift beer fridge at the bottom of the garden, feeding from the outside building. Anyone who comes into contact with the refrigerator while standing barefoot on damp lawn has a good chance of feeling an unpleasant tingling sensation from the potential difference. This is because any stray earth leakage from the refrigerator takes the path of least resistance, and in this case, it is through the wet body to the near ground position, which is not necessarily the installation ground, only the one on the system, see Figure 1. PEN conductor – open circuit is a concern Most of the perceived shocks are the risks associated with losing the neutral on the supply. This can be caused by a breakdown of the underground cable joint or possibly the separation of the neutral from an overhead source, for example, on a utility shaft by a fallen tree. The problem is that if the line conductor remains connected, the returning load current will not be able to return to the source due to the loss of the neutral, and because both the neutral and the ground are connected to the service head, the load current will now seek an alternate return. Consequently, the probability of all exposed conductive parts and external conductive parts connected to the installation of small and medium equipment will increase, resulting in the risk of electric shock. The isoelectric protective bonding will limit the impacts within the isoelectric region. However, it is out of a different story, and it could be fatal. TT system for outbuildings as evidenced by the aforementioned concerns, many electricians will have to make an engineering judgment about the likelihood of a potential problem based on all the factors presented to them on the job. If PME supplies were used for lighting fixtures in a wooden shed where all fittings were plastic and the shed had a wooden floor, the probability of a problem occurring would be minimal. On the other hand, if the shed has socket outlets that can be used for outdoor equipment, a potential problem could arise. It should be noted that incidents related to loss of the PEN conductor are few and far between. In fact, the same shock risk would apply to any PME installation where the neutral supply has been lost and there is a metal exterior faucet connected to the grounding arrangement by protective connection. For any outdoor facilities where there is a concern about using PME supplies from the main building, it is recommended to convert the outdoor building to a TT system. This includes using a local ground pole rather than using the land from the main building. The PME grounding must be disconnected at the outdoor construction site. This can be accomplished with an adaptive insulated box, or by an insulating gland to separate grounding systems. Figure 2 shows an example of a separate workshop with a TT grounding arrangement. This allows the overcurrent protection device in the main building to operate in the event of a ground line fault on the supply cable. A local RCD provides fault protection for circuits within the workshop. It is worth marking inside the adaptive box to ensure that this intentional separation of grounding systems in the future is not reconnected by a less experienced electrician – see Figure 3 for an example of the label. Conclusion We hope this article has demonstrated that there is not always a definitive answer to these design options and contractors should bear this in mind when carrying out EICR work, as some outbuildings may use PME with few associated risks. When in doubt, use a TT grounding system to avoid any potential issues. Get more details on registering for the NAPIT scheme by clicking here


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