Proper grounding ensures that the protection devices that are installed for personal safety and equipment protection will operate in the manner desired. Grounding is also an important aspect for your electrical service since it provides a stable electrical reference point.
Listed below is a brief list of some of the important features of an effective grounding electrical system.
- Permanent and continuous ground wire.
- Capacity to conduct a circuit's maximum fault current.
- Provides a low impedance path to ground for proper operation of circuit protective devices.
- Uses more than just the earth as the equipment's ground conductor.
- Grounding rod should have a low resistance of 5 ohms or less.>
- Neutral point of your electrical system should be connected to the grounded conductor at the service entrance. This point should also be connected to the ground conductor via the bonding jumper.
- Ground conductor should be connected to the building's grounding electrode via the grounding electrode conductor at the service entrance.
- Individual branch circuits are utilized to power sensitive equipment.
- Conduits are not used as the sole source for grounding sensitive equipment.
- Green wire grounds should be of the same size as the current carrying conductors.
- Individual circuit conduit should be bonded at both ends.
- Building steel should be used as a low impedance ground reference.
- Building steel should be well bonded and the main vertical columns should be earth-grounded as well.
A diagram of which shows the basic elements of a properly grounded electrical system is shown below.
(Reference: National Electrical Code, Section 250) (Photo courtesy of Kansas City Power and Light)
A common power quality problem that occurs due to a poorly designed or installed grounding system is electrical noise or electromagnetic interference (EMI). Noise can cause disruptions in control system circuits, which may stop or interfere the proper operation of equipment in your facility. EMI can also cause lighting restarts in high intensity discharge (HID) lighting systems and light flicker in fluorescent lighting using electronic ballasts.
The following general grounding guidelines will provide you with a means for electrical noise reduction in a facility:
- To improve the noise performance of electrical circuits with sensitive loads, use isolated ground receptacles. You can identify isolated ground receptacles by their orange coloring, or some may have an orange triangle located on the face of the receptacle (per NEC, Section 410-56c).
- If the frequency response of the grounding system is very important (i.e. computer room), install a signal reference grid under a raised floor or use the raised floor as signal reference grid. (NOTE: This is not a replacement for the safety ground, but augments the safety ground in order to reduce noise).
For long electrical branch circuits, it is often difficult to limit the neutral-to-ground voltage. In this case, consider a separately derived system that has a ground reference independent of other systems. A common example is an isolation transformer that allows the neutral and ground to be bonded together to establish a new ground reference. Remember that any separately derived system should have only one neutral-to-ground bond. An example of a separately derived system is shown below.
(Reference: National Electrical Code, Section 250-26) (Photo courtesy of Kansas City Power and Light)
Please note that the current edition of the National Electrical Code (NEC) is always the best reference for answering any grounding application questions that you may have.