6 wiring and grounding problems that lead to low power quality

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Wiring and grounding problems

In this technical article, typical wiring and grounding problems, as related to power quality, are presented. Possible solutions are given for these problems as well as the possible causes for the problems being observed on the grounding system. (See Table 2 at the bottom of article)

wiring grounding problems 1

6 wiring and grounding problems that lead to low power quality

The following list is just a sample of problems that can occur on the grounding system.

  1. Isolated grounds
  2. Ground loops
  3. Missing safety ground
  4. Multiple neutral-to-ground bonds
  5. Additional ground rods
  6. Insufficient neutral conductors

1. Insulated grounds

Insulated grounds in themselves are not a grounding problem. However, improperly used insulated grounds can be a problem. Insulated grounds are used to control noise on the grounding system. This is accomplished by using insulated ground receptacles, which are indicated by a “∆” on the face of the outlet.

Insulated ground receptacles are often orange in color. Figure 1 illustrates a properly wired insulated ground circuit.
Properly wired isolated ground circuit

2properly wired isolated ground circuit 768x404
Figure 1 – Properly wired isolated ground circuit

The NEC has this to say about insulated grounds.

NEC 250-74 Connecting receptacle grounding terminal to box

An equipment bonding jumper shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box.

Exception No. 4. Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted. The receptacle grounding terminal shall be grounded by an insulated equipment grounding conductor run with the circuit conductors. This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in Section 384-20, Exception so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or source.

(FPN): Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system and outlet box.

 

NEC 517-16 Receptacles with insulated grounding terminals

Receptacles with insulated grounding terminals, as permitted in Section 250-74, Exception No. 4, shall be identified. Such identification shall be visible after installation.

(FPN): Caution is important in specifying such a system with receptacles having insulated grounding terminals, since the grounding impedance is controlled only by the grounding conductors and does not benefit functionally from any parallel grounding paths.

 

The following is a list of pitfalls that should be avoided when installing insulated ground circuits:

  • Running an insulated ground circuit to a regular receptacle.
  • Sharing the conduit of an insulated ground circuit with another circuit.
  • Installing an insulated ground receptacle in a two-gang box with another circuit.
  • Not running the insulated ground circuit in a metal cable armor or conduit.
  • Do not assume that an insulated ground receptacle has a truly insulated ground.

 

2. Ground loops

Ground loops can occur for several reasons. One is when two or more pieces of equipment share a common circuit like a communication circuit, but have separate grounding systems (Figure 2).

3circuit with ground loop
Figure 2 – Circuit with a ground loop

To avoid this problem, only one ground should be used for grounding systems in a building. More than one grounding electrode can be used, but they must be tied together (NEC 250-81, 250-83, and 250-84) as illustrated in Figure 3 below.
4grounding electrodes bonded together
Figure 3 – Grounding electrodes must be bonded together

 

3. Missing safety ground

A missing safety ground poses a serious problem. Missing safety grounds usually occur because the safety ground has been bypassed. This is typical in buildings where the 120-volt outlets only have two conductors.

 

Modern equipment is typically equipped with a plug that has three prongs, one of which is a ground prong. When using this equipment on a two-prong outlet, a grounding plug adapter or “cheater plug” can be employed provided there is an equipment ground present in the outlet box.

 

This device allows the use of a three-prong device in a two-prong outlet. When properly connected, the safety ground remains intact. Figure 4 illustrates the proper use of the cheater plug.

5grounding plug adapter

Figure 4 – Proper use of a grounding plug adapter or “cheater plug”

If an equipment ground is not present in the outlet box, then the grounding plug adapter should not be used. If the equipment grounding conductor is present, the preferred method for solving the missing safety ground problem is to install a new three-prong outlet in the outlet box.

This method insures that the grounding conductor will not be bypassed. The NEC discusses equipment grounding conductors in detail in Section 250 — Grounding.

 

4. Multiple neutral to ground bonds

Another misconception when grounding equipment is that the neutral must be tied to the grounding conductor. Only one neutral-to-ground bond is permitted in a system or sub-system. This typically occurs at the service entrance to a facility unless there is a separately derived system.
A separately derived system is defined as a system that receives its power from the windings of a transformer, generator, or some type of converter. Separately derived systems must be grounded in accordance with NEC 250-26.

 

The neutral should be kept separate from the grounding conductor in all panels and junction boxes that are downline from the service entrance. Extra neutral-to-ground bonds in a power system will cause neutral currents to flow on the ground system.

 

This flow of current on the ground system occurs because of the parallel paths. Figures 5 and 6 illustrate this effect.

6neutral current flow
Figure 5 – Neutral current flow with one neutral-to-ground bond

neutral current flow extra neutral to ground bond
Figure 6 – Neutral current flow with and extra neutral-to-ground bond

As seen in Figure 6, neutral current can find its way onto the ground system due to the extra neutral-to-ground bond in the secondary panel board. Notice that not only will current flow in the ground wire for the power system, but currents can flow in the shield wire for the communication cable between the two PCs.

If the neutral-to-ground bond needs to be reestablished (high neutral-to-ground voltages), this can be accomplished by creating a separately derived system as defined above. Figure 7 illustrates a separately derived system.
7separately derived system
Figure 7 – Example of the use of a separately derived system

 

5. Additional ground rods

Additional ground rods are another common problem in grounding systems. Ground rods for a facility or building should be part of the grounding system. The ground rods should be connected where all the building grounding electrodes are bonded together.

Isolated grounds can be used as described in the NEC’s Isolated Ground section, but should not be confused with isolated ground rods, which are not permitted.

 

The main problem with additional ground rods is that they create secondary paths for transient currents, such as lightning strikes, to flow. When a facility incorporates the use of one ground rod, any currents caused by lightning will enter the building ground system at one point.The ground potential of the entire facility will rise and fall together.

 

However, if there is more than one ground rod for the facility, the transient current enters the facility’s grounding system at more than one location and a portion of the transient current will flow on the grounding system causing the ground potential of equipment to rise at different levels.

This, in turn, can cause severe transient voltage problems and possible conductor overload conditions!

 

6. Insufficient neutral conductor

With the increased use of electronic equipment in commercial buildings, there is a growing concern for the increased current imposed on the grounded conductor (neutral conductor). With a typical three-phase load that is balanced, there is theoretically no current flowing in the neutral conductor, as illustrated in Figure 8.

8balanced three phase system 768x534
Figure 8 – A balanced three-phase system

However, PCs, laser printers, and other pieces of electronic office equipment all use the same basic technology for receiving the power that they need to operate. Figure 9 illustrates the typical power supply of a PC. The input power is generally 120 volts AC, single phase.

The internal electronic parts require various levels of DC voltage (e.g., ± 5, 12 volts DC) to operate.
9one line smps 768x328
Figure 9 – The basic one-line for a SMPS

This DC voltage is obtained by converting the AC voltage through some type of rectifier circuit as shown. The capacitor is used for filtering and smoothing the rectified AC signal. These types of power supplies are referred to as switch mode power supplies (SMPS).

The concern with devices that incorporate the use of SMPS is that they introduce triplen harmonics into the power system.

Triplen harmonics are those that are odd multiples of the fundamental frequency component (h = 3, 9, 15, 21, …). For a system that has balanced single-phase loads as illustrated in Figure 10, fundamental and third harmonic components are present.

Applying Kirchoff’s current law at node N shows that the fundamental current component in the neutral must be zero. But when loads are balanced, the third harmonic components in each phase coincide. Therefore, the magnitude of third harmonic current in the neutral must be three times the third harmonic phase current.
10balanced single phase loads 768x533

This becomes a problem in office buildings when multiple single-phase loads are supplied from a three-phase system. Separate neutral wires are run with each circuit, therefore the neutral current will be equivalent to the line current.

However, when the multiple neutral currents are returned to the panel or transformer serving the loads, the triplen currents will add in the common neutral for the panel and this can cause over heating and eventually even cause failure of the neutral conductor!

If office partitions are used, the same, often undersized neutral conductor is run in the partition with three-phase conductors. Each receptacle is fed from a separate phase in order to balance the load current.

NOTE! However, a single neutral is usually shared by all three phases. This can lead to disastrous results if the partition electrical receptacles are used to supply nonlinear loads rich in triplen harmonics. Under the worst conditions, the neutral current will never exceed 173% of the phase current.


Figure 10 illustrates a case where a three-phase panel is used to serve multiple single-phase SMPS PCs.

 

Summary

As discussed above, the three main reasons for grounding in electrical systems are:

  1. Personal safety
  2. Proper protective device operation
  3. Noise control

By following the guidelines found below, the objectives for grounding can be accomplished:

  • All equipment should have a safety ground. A safety ground conductor
  • Avoid load currents on the grounding system.
  • Place all equipment in a system on the same equipotential reference.

Table 1 summarizes typical wiring and grounding issues.

Table 1 – Summary of wiring and grounding issues

Good power quality and noise control practices do not conflict with safety requirements.
Wiring and grounding problems cause a majority of equipment interference problems.
Make an effort to put sensitive equipment on dedicated circuits.
The grounded conductor, neutral conductor, should be bonded to the ground at the transformer or main panel, but not at other panel down line except as allowed by separately derived systems.

Table 2 – Typical wiring and grounding problems and causes

Wiring Condition or Problem Observed 

Possible Cause

Impulse, voltage drop out Loose connections
Impulse, voltage drop out Faulty breaker
Ground currents Extra neutral-to-ground bond
Ground currents Neutral-to-ground reversal
Extreme voltage fluctuations High impedance in neutral circuit
Voltage fluctuations High impedance neutral-to-ground bonds
High neutral to ground voltage High impedance ground
Burnt smell at the panel, junction box, or load Faulted conductor, bad connection, arcing, or overloaded wiring
Panel or junction box is warm to the touch Faulty circuit breaker or bad connection
Buzzing sound Arcing
Scorched insulation Overloaded wiring, faulted conductor, or bad connection
Scorched panel or junction box Bad connection, faulted conductor
No voltage at load equipment Tripped breaker, bad connection, or faulted conductor
Intermittent voltage at the load equipment Bad connection or arcing


 

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See the origial article at:  http://electrical-engineering-portal.com/6-wiring-grounding-problems



 

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