Equipotential bonding is essential for maintaining safety and system integrity in electrical installations. However, one frequent mistake involves looping the earth around multiple cable glands without properly terminating it to an earth point.
This practice not only poses risks like circulating currents and heat build up but can also lead to significant electromagnetic compatibility (EMC) issues. Let’s explore these risks in depth, including how EMC problems arise, the symptoms they can cause, and outline best practices to avoid them.
Of course time tested 7671 engineers are aware of the this subject. This is more written for those who are not aware of the destructive forces present if not addressed and implemented. If you are a contractor reading this just take on board the seemingly simplistic and obvious, then factor in real life and timelines.
Providing the fundamentals are addressed not a problem. If not get ready for for some odd behaviour from your plant, not to mention safety issues.
Escalating Symptoms – Not a myth!
When the load on a system with improper earth bonding increases to its maximum, the severity of the issues escalates dramatically. The higher current flow in each phase generates stronger electromagnetic fields, which induce even greater circulating currents within the looped earth conductor.
These currents can lead to significant overheating of the cable glands, posing an immediate fire hazard. Additionally, the increased load exacerbates phase imbalances, causing severe harmonic distortion. This distortion can disrupt the operation of sensitive equipment, lead to voltage fluctuations, and result in erratic performance or even shutdowns of connected devices.
In extreme cases, protective devices may fail to operate correctly, prolonging fault conditions and increasing the likelihood of catastrophic equipment failure or electrical fires. As switchgear and controls we take pride in making the kit. When it leaves and lands in on site the installers then get to connect and fulfil the service. Providing your installers take the necessary steps great.
I would rather state the obvious than not, been here seen the installer wearing the t-shirt.
The Scenario: Improper Earth Bonding
In this scenario, one building houses a low-voltage (LV) board where the cable glands are correctly earthed. However, at the opposite end—where the cables terminate in another building—the glands are looped together with an earth conductor but are not properly grounded.
This setup effectively creates a floating loop at the ungrounded end, which becomes problematic when exposed to the electromagnetic fields generated by the separate phases running through the cables. Even though the glands at the LV board are correctly earthed, the looped glands at the other end allow for circulating currents, which can result in excessive heat build up at the glands, phase imbalances, and electromagnetic compatibility (EMC) challenges.
The Dangers of Circulating Currents
1. Heat Generation
The looped earth acts as a secondary conductor. Electromagnetic fields from different phases induce circulating currents within the loop, generating heat. Over time, this heat can degrade cable insulation, damage components, and, in severe cases, lead to fire hazards.
2. Impaired Fault Protection
Without a low-impedance path to ground, fault currents may not clear effectively. This delays the operation of protective devices like circuit breakers, increasing the risk of prolonged fault conditions.
3. Energy Loss
Circulating currents result in unnecessary energy dissipation as heat, reducing system efficiency and increasing operational costs.
Symptoms of Earth Loop Issues
In a real-world scenario, where only one end of the cables was correctly earthed while the other had looped earth conductors, the following symptoms were observed:
Odd Phasing and Load-Induced Noise
When a load was applied, the system exhibited phase imbalances and audible noise, often described as humming or buzzing. This noise is caused by harmonic distortions and interference resulting from circulating currents in the looped earth conductor.
Overheating of Cable Glands
The circulating currents within the looped earth generated significant heat, causing the cable glands to become excessively hot. This heat buildup poses risks to the integrity of the installation, potentially damaging components and creating a fire hazard.
These issues directly correlate with earth loop problems, where induced currents disrupt the normal operation of the system and compromise safety.
How EMC Issues Arise
Improper bonding introduces EMC challenges that can compromise the performance of sensitive equipment:
1. Induced Currents from Electromagnetic Fields
In a three-phase system, each phase generates its own electromagnetic field. When a looped earth conductor is exposed to these fields, circulating currents are induced, turning the loop into a source of electromagnetic interference (EMI). This interference can disrupt nearby equipment, particularly in environments with sensitive electronics.
2. Ground Loops
Ground loops form when there are multiple paths to earth with different potentials. The looped earth conductor creates such a path, causing low-frequency noise. This is especially harmful to systems with communication networks or control circuits, where even small voltage differences can lead to data corruption or erratic behavior.
3. Resonance Effects
Depending on its size and geometry, the loop can act as an antenna, resonating at specific frequencies. This amplifies high-frequency noise, which can interfere with devices like programmable logic controllers (PLCs) or industrial Ethernet systems, leading to malfunction or failure.
Real-World EMC Impacts
1. Communication Failures
Improper bonding has led to noise interference in industrial Ethernet networks, causing packet loss and intermittent connectivity issues. This disrupted critical operations, requiring costly troubleshooting and downtime.
2. Control Panel Malfunctions
Circulating currents induced by improper bonding caused erratic behavior in motor control circuits, triggering false alarms and unnecessary shutdowns.
3. Instrument Accuracy Compromise
Sensitive instrumentation, such as flow meters and temperature sensors, experienced fluctuating readings due to EMI. This compromised process control and led to inefficiencies in production.
Best Practices for Proper Bonding
1. Separate Earth Conductors for Each Gland
Preferred Method: Run individual earth conductors from each cable gland to a dedicated earth point. This provides a direct, low-impedance path to ground, eliminating the risk of circulating currents and ensuring effective fault protection.
2. Looping with Proper Earth Termination
If looping the earth is unavoidable, ensure the loop is terminated to an earth point to break the closed loop and prevent circulating currents.
3. Use Insulated Glands Where Necessary
In systems where full bonding isn’t required, insulated glands can prevent unintended connections across phases, reducing the risk of EMC issues.
4. Shielding and Reducing Loop Area
Use shielded cables in high-EMI environments to contain electromagnetic fields. If loops are necessary, minimise their size to reduce susceptibility to induced currents and resonance.
5. Perform EMC Testing
Conduct EMC testing during commissioning to identify potential interference. Tools like spectrum analysers and thermal imaging can help pinpoint problem areas and validate the effectiveness of bonding. This is more post actions should you discover the worst or random things start going bump in the night.
Better Safe Than Sorry
Improper bonding of cable glands poses significant risks, from circulating currents and heat build up to severe EMC issues that disrupt system performance. Odd phasing, audible noise under load, and overheating are clear symptoms of earth loop problems.
By adopting best practices—such as running separate earth conductors or ensuring proper termination of loops—you can eliminate these risks and ensure safe, reliable operation.
Disclaimer:
The information provided on this site is for general informational purposes only and may not reflect the most current regulations or standards. Legislation, industry guidelines, and best practices can change over time, and it is the user’s responsibility to research and ensure compliance with the latest requirements for their specific situation. Always consult a qualified professional for advice tailored to your project or application.