Your facility, technology and other key assets are at risk to lightning strikes. While an effective air termination device and above ground lightning protection system is critical to effective protection, proper grounding can keep people and products out of harm’s way.
Preventable mistakes in the electrical grounding system can leave it vulnerable to ground faults, side flashes and electronic noise, any of which could result in fire, damage to structures and personal injury.
Below we outline 5 common mistakes made when designing and installing a grounding system.
1. Not Understanding Resistivity and Impedance Guidelines
The primary goal of a grounding system is to provide a low impedance route to the earth.
Involved in that process is the earth—or soil—itself. Soil resistivity is an important design consideration that determines the starting point in designing any grounding system. It varies significantly for different soil types, moisture content levels and temperatures—each characteristic gives rise to variations in ground impedances.
High resistance earth provides unsafe paths for the fault current, increasing the risk of equipment failures and injury caused by step and touch potential. Current will always follow the path of least resistance; therefore, if low impedance grounding is not in place, the alternate path to ground may include sensitive equipment or the human body.
The table below provides some resistivity values of common soils found throughout the world:
Safety cannot be compromised due to an ineffective grounding system. Understanding impedance guidelines is crucial to designing and installing an effective system.
Review grounding codes and standards, principles and more in the nVent ERICO Grounding and Bonding and nVent ERICO Cadweld Solutions Guide.
2. Failing to Measure the Grounding System After Install
Once grounding systems are fully designed, specified and implemented, it’s important to check that variances were not found after install, as a variance can occur that was not initially intended. For instance, variances may occur if a ground rod hits a large rock underground and does not reach its desired depth. These variances can cause a much higher ground resistance than initially desired.
The best way to guarantee that the grounding system has a sufficient resistance is to measure that resistance using the three-point method prior to connecting the system to the utility.
The National Electric Code (NEC) requirements for most applications require an additional ground rod if the first rod driven measures greater than 25 ohms. Even though the NEC does not require additional measurement or procedures after the second ground rod, if the resistance is still very high, this may cause problems.
3. Not Reattaching Ground Paths After Relocating, Replacing or Removing Equipment for Repairs
All connections of the ground path, including setscrews, locknuts and threads, should be fully engaged and tested for continuity before putting the system back into service. Check for dirt, rust and corrosion because such impurities can compromise the metal raceway.
Without proper securing of the line, the low-impedance circuit will be impaired, increasing the risk of faults. Ensure the entire path is reconnected completely after repairs have been finished.
4. Using Ground Wire Incorrectly
When installing ground wire or inspecting a grounding system, it is important to pay attention to the conductor bends and lengths. While most electricians may leave excess coils of wire inside of panels for future maintenance, ground wire should not be subjected to surplus lengths. Ground wire that is bunched up or coiled together creates high impedance paths via mutual inductance, which will cause higher voltages to be seen on equipment during transient events. The inductance that these coils generate cannot be measured using a DC meter, as they are AC-based impedances.
Refrain from coiling the wire because the number one rule of grounding is to provide the current a straight, simple route to ground. Be sure to use gradual bends, short wire lengths and secure connections when installing ground wire.
5. Choosing Incorrect Ground Wire
Low impedance to ground is fundamental to the system. When choosing wire as a down conductor, choose thicker gauge wire. Thicker wire exhibits inherently lower resistance, allowing an easier path for the current to the ground. The size of the ground wire should pertain directly to the short-circuit current ratings of the system, and the overcurrent protection devices in use. While the NEC gives no maximum impedance of the ground-fault circuit, it does however state in 250.4(A)(5) that it shall be sufficiently low to facilitate the operation of the circuit-protective devices.
All manufacturers of circuit breakers or fuses release time-current curves on products, which allows for specifiers to correctly choose the ground wire size to generate the adequate amount of current necessary. Some engineering designs may incorporate the concept of the fault current being no less than five times the rating of the overcurrent device. For example, if the overcurrent device is 400A, there needs to be no less than 2000A of current in the circuit for it to open the faulted circuit within a reasonable time.
Want to Learn More About Proper Grounding, From Down Conductors to Ground Rods?
The nVent ERICO Grounding and Bonding and nVent ERICO Cadweld Solutions Guide provides comprehensive information on the principals of a proper grounding system, the importance of such a system, and the highest-grade products available to create a dependable grounding system.
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