Forums » Electrical Engineering

two point grounding system

    • 19 posts
    April 24, 2019 1:53 PM PDT

    I have looked that in a substation the equipment have been grounded or bonded to ground from two points. In some other case, there is a redundant path connected for grounding or bonding for equipment. If one fails, the second works.

    But there is one more case, where more than one point connection is limited only to one point due to circulating fault currents as an example two points bonded cable sheaths.

    Does anyone have experience to find out the best option?
    What NFPA recommends ?

    • 200 posts
    June 16, 2019 5:19 PM PDT

    Thanks for the suggestion of this topic, Mercado. Sorry for the late reply.

    About circulating fault currents, I suggest a reading on "MCCB switchgear" topic here in Z4E Forum. It is at:
    https://zone4engineer.com/forums/topic/31/mccb-switchgear

    There, it is possible to realize how difficult is to design a multiple path ground system.

    Multiground (as referred by NEC/NFPA) is also called Multipoint Ground (as referred by U.S. Military standards) or Equipotential Ground. However many of Engineers and Technicians, including myself, like to make a distinction to Equipotential Ground for the grounding system with a "grounding mesh" or an "equipotential mesh", there are many paths between any two ground points. Multiground or Multipoint Ground is often (publicly) related to more than one ground point, each point connected to a grounding cable usually in star topology. But it is technically correct consider them all the same grounding method.

    Equipotential Ground (mesh) is better than a Multiground (star topology) in wider range of frequencies, mainly high frequencies designs (greater than 30KhZ), with special calculations related to the wavelength of the highest frequency.

    Equipotential Ground (mesh) is also applied to swimming pools or any portions of water used by humans - involving metals and electrical equipment -, as well in electrical transmission towers and external areas of industrial plants and stations including metal fences - mainly related to lightning protection.

    The Multiground (star topology) is mainly used on 50 or 60 Hz plants and stations design.

    If there are large electrical motors (and generators), they should also be on the multipoint grid, but NOT on the same segment of the grounding system serving sensitive electrical equipment and electronic devices.

    Multiground is a "design consideration" according to the application, it is not a NEC/NFPA recommendation - however if Multiground is a necessity then NEC/NFPA requires "to do the right way", with special concern on Multigrounded Neutral Systems, as can be found in ARTICLE 250 — GROUNDING AND BONDING, 250.184 Solidly Grounded Neutral Systems, (C) Multigrounded Neutral Systems. The size of the grounding conductors is ruled local electrical code, in U.S. by NEC/NFPA.

    The NEC/NFPA requires that equipment-grounding conductors to ground the exposed metal surfaces. It becomes difficult to do this with a third conductor in a way that does not result in multiple point connections between the negative, current-carrying conductor and the grounding system - maybe that is the case you found. Instead of multiple point grounding, other solutions can be applied such as: non-metallic enclosures to isolate the grounded chassis and ground-isolated connections for other sensitive equipment.

    Multiground is an alternative type of electrical ground mainly to solve these problems:

    - ground loop or earth loop: two points of a circuit both intended to be at ground reference potential have a potential between them. Ground loops are a major cause of noise, hum, and interference (see below);
    - power line hum, a noise associated with AC power usually at double of fundamental 50 Hz or 60 Hz, meaning 100hz or 120hz depending on the local frequency, with heavy harmonic content;
    - Lightning protection;
    - for the REDUNDANCY CASE as you found in a substation: maybe it is limited to an specific equipment with high risk of Ground Fault (GF), equipment installed on two separated skids, or even this very equipment is a source of noise and interference...

    ... but the usual installation, despite not always, is only one ground point with multiple ground cables connected to it for a system (group of equipment connected altogether) of even for the entire plant or station, if more related to prevent Ground Faults than lightning protection.

    The connection of the equipment-grounding conductor can run from module to module and then to the switchgear and power center. The order of the connections is NOT critical and multiple connections or parallel connections usually do not cause problems. Each equipment-grounding conductor may also be run from the metal surface being grounded to a central point like the power center.

    Multiple connections to one grounding electrode conductor refer to connections from the power system and do NOT refer to instrument, computer or communication grounds. Multiple connections to one grounding electrode may also occur when several ground rods are bonded together to form a grounding electrode system, and when metal water pipes or well casings are bonded to the ground rod. Multiple connections are also common where DC and AC grounding electrode conductors are connected to the SAME ground rod. Several equipment-grounding conductors tied to the one ground rod make no need to use small grounding electrode conductor.

    Usually all equipment-grounding conductors should be connected to a bus bar. If there are multiple grounding electrodes in the system, the secondary electrodes should all be connected to this bus bar to complete the grounding electrode system. Meaning: the bus bar makes simulates a single-point ground as related to equipment.

    This reply is NOT a definitive answer, just additional information. Grounding is an extensive and difficult subject which can lead to many discussions and a great deal of different information..., this is not the end...

    Regards


    This post was edited by Alex de Moura at June 16, 2019 5:21 PM PDT