Grounding System And Article 250 Of The NEC
Earthing an electricity supply network necessitates the network plant as well as the customer electrical equipment to both be connected to the ground. This is done to promote safety and also to reduce probable equipment damage.
The most effective earthing systems prevent over voltages as they also minimize the risk of electric shock. An earthing system works as it provides a predetermined route for the leakage currents. It is a system that disconnects the faulty circuit or plant by activating protective devices.
There are three different types of earthing systems –
The low voltage or LV distribution system is identifiable or defined using five letters which are T for direct connection to the ground; N for neutral; C for combined; S for separate and I for isolated from the ground or earth. The initial letter shows the supply source which is earthed and the second letter depicts the frame or how the metalwork is earthed. The next two letters denote the functions of protective and neutral conductors.
There are three probable configurations which are –
- TT or transformer neutral earthed as well as frame earthed.
- IT or unearthed transformer neutral; earthed frame.
- TN or transformer neutral earthed, with the frame linked to neutral. This can be further categorized into TN-C-S; TN-C or TN-S.
Earthing systems are one of the first things that you need to carefully consider when setting up an electrical system.
Going back to the letters on the system, always remember the first letter to be the connection between the power supply (energy source) and the earth (ground). An example is a transformer or generator.
It is crucial that you choose the correct kind of electrical earthing system from the onset. Your choice will eventually affect the electromagnetic compatibility and safety of the power supply.
It is also important to provide a PE or protective earth connection. This will ensure that every exposed conductive surface has an equal electrical potential as that of the earth’s surface, hence, this prevents any person from touching the device during short circuit, averting electric shock or electrocution in the process. The FE or function earth connection is utilized for devices like antennas. It may carry some current during regular operations.
The Contractor’s Role
The electrical contractor is a professional that is given the task to install basic to complex electrical systems. He and his team are there to install grounding systems, also known as earthing systems. To do this, the team must know all the relevant standards and codes in putting up earthing systems. The task becomes more difficult as the facilities that they design earthing systems on get bigger. For larger facilities, the earthing system project is often left to the hands of a qualified electrical engineer.
Guesswork simply won’t work when it comes to installing earthing systems. This is why Article 250 of the NEC or National Electrical Code was created. This article contains the universal requirements for bonding and grounding electrical installations in all types of structures from residential, commercial to industrial establishments in the U.S.
An example of this is which conductors should be grounded; what are the proper locations for grounding connections; which systems should be grounded or ungrounded; the correct size of grounding and bonding conductors, etc. The bottom-line always is the user’s safety as grounding protects him from potential fatal injuries and shocks. Article 250, just like the rest of NEC, is constantly updated to keep up with the changing technology and methods. It is important to be abreast with such updates in order to eliminate non-compliance. If you are not sure about what to do, it is best to leave the project to the experts.
Related links
https://fyi.extension.wisc.edu/mrec/files/2011/04/W4.-Biesterveld-NEC-grounding-MREC2010.pdf
https://www.nfpa.org/NEC/About-the-NEC/Grounding-and-bonding
http://www.codebookcity.com/codearticles/nec/necarticle250.htm
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