Eddy currents are a hidden but potentially dangerous issue in electrical installations, particularly when single conductors pass through individual holes in ferrous gland plates.
Using a ferrous place creates ideal conditions for circulating currents, which generate heat and can lead to inefficiencies, equipment failure, and increased maintenance costs. Moreover, eddy currents can contribute to electromagnetic compatibility (EMC) problems, further complicating system reliability and potentially interfering with nearby sensitive equipment.
This page provides an overview of eddy currents, their effects, and strategies for mitigating them in ferrous gland plates. We’ll also explore the benefits of non-ferrous gland plates and provide thickness recommendations for both materials based on current levels and application types.
What Are Eddy Currents and Why Are They a Problem?
Eddy currents are loops of electrical current induced in conductive materials by changing magnetic fields. In ferrous gland plates:
Alternating magnetic fields from conductors induce circulating currents. These eddy currents flow in loops around the conductor holes.
The circulating currents generate heat, leading to:
- Energy loss.
- Overheating of the gland plate.
- Degradation of insulation and materials.
The Negative Effects of Eddy Currents
- Heat Generation:
The most immediate effect of eddy currents is localized heat generation. Over time, this can cause:- Thermal stress on the plate and nearby components.
- Potential hot spots leading to equipment failure.
- Energy Loss:
The energy that feeds these currents is essentially wasted, reducing the overall system efficiency. - Material Degradation:
Prolonged heating can weaken the structural integrity of the gland plate, degrade conductor insulation, and accelerate corrosion. - Electromagnetic Compatibility (EMC) Issues:
Eddy currents can generate unwanted electromagnetic interference (EMI). This interference can:- Disrupt the operation of nearby sensitive electronic equipment.
- Cause signal degradation or data loss in communication systems.
- Compromise the performance of control systems and sensors, leading to operational inefficiencies or unsafe conditions.
- Increased Maintenance and Downtime:
Overheating and material wear can lead to frequent maintenance needs and costly downtime. - Safety Risks:
Overheating increases the likelihood of fire hazards and catastrophic equipment failure, posing risks to both personnel and property.
Why Non-Ferrous Gland Plates Are Different
Unlike ferrous plates, non-ferrous gland plates (e.g., aluminum or plastic):
- Do not support eddy current formation due to their low magnetic permeability.
- Generate minimal heat from electromagnetic fields, even in high-current installations.
- Can be thinner while maintaining the same level of safety and performance, since they don’t require extra material thickness to mitigate heat build up.
How Cutting Channels Mitigates Eddy Currents in Ferrous Plates
The key to mitigating eddy currents in ferrous plates is to interrupt the conductive loops by cutting channels between the holes. This breaks the path that allows eddy currents to flow, significantly reducing heat generation and minimizing EMC issues.
Mathematics Behind Channel Width Calculations
To determine the required channel width, we need to account for two factors:
- Skin Depth (δ)
- Gland Plate Thickness
What is Skin Depth?
Skin depth is the distance within a conductive material where the current density decreases to approximately 37% of its value at the surface. It indicates how deeply alternating current (AC) penetrates the material. Beyond this depth, the current is significantly reduced, and the material is less affected by electromagnetic fields.
Why is Skin Depth Important?
In the context of eddy currents:
- Most eddy currents are confined within the skin depth of the material.
- For steel at 50 Hz, the skin depth is approximately 8.5 mm.
- This means that eddy currents primarily flow within the first 8.5 mm of the material’s surface. If the gland plate is thicker than this, the excess thickness does not contribute significantly to the eddy currents but still adds structural weight.
Implications for Channel Width:
Since eddy currents are mostly confined to the skin depth, the effective channel width needs to disrupt these currents over at least this depth. That’s why the formula for channel width takes both skin depth and plate thickness into account:
Step 1: Calculate Skin Depth
Skin depth is the distance within a conductive material where most of the current flows. It depends on the material’s properties and the frequency of the current.
Where:
μ = Magnetic permeability of the material. For steel:
σ = Electrical conductivity of the material. For steel:
ω=2πf, with f=50 Hzf = 50 \, \text{Hz}f=50Hz:
For steel at 50 Hz: Skin depth depends only on the material properties (μ, σ) and the frequency (f), not the physical thickness of the gland plate.
Step 2: Adjust for Gland Plate Thickness
If the gland plate thickness 
Effective Channel Width Calculation:
Where:
Thickness of the gland plate:
Required channel width:
Realistic Gland Plate Thickness for Ferrous Plates
The thickness of ferrous gland plates varies depending on the application and current levels:
| Application | Typical Gland Plate Thickness (Ferrous) |
|---|---|
| Light Commercial (<100A) | 3 mm – 5 mm |
| Medium Commercial (100A–400A) | 5 mm – 8 mm |
| Industrial (400A–1000A) | 8 mm – 12 mm |
| Heavy Industrial (>1000A) | 12 mm – 15 mm |
For non-ferrous plates, these thicknesses can often be reduced significantly because eddy currents are not a concern.
Practical Channel Width Recommendations for Ferrous Plates
Here’s an updated table for channel widths based on typical hole sizes, conductor sizes, and ferrous plate thickness:
| Hole Diameter (mm) | Typical Conductor Size | Gland Plate Thickness (mm) | Recommended Channel Width (mm) |
|---|---|---|---|
| 20 mm | 2.5 mm² to 6 mm² | 3 mm – 5 mm | 8 mm – 10 mm |
| 25 mm | 10 mm² to 16 mm² | 5 mm – 6 mm | 10 mm |
| 32 mm | 25 mm² to 35 mm² | 6 mm – 8 mm | 11 mm – 12 mm |
| 40 mm | 50 mm² to 70 mm² | 8 mm – 10 mm | 12 mm |
| 50 mm | 95 mm² to 120 mm² | 10 mm – 12 mm | 14 mm |
| 63 mm | 150 mm² to 185 mm² | 12 mm | 14 mm |
| 75 mm | 240 mm² to 300 mm² | 12 mm – 15 mm | 15 mm – 18 mm |
Predicted Thickness Requirements for Non-Ferrous Plates
Non-ferrous plates don’t require the same thicknesses as ferrous plates. Here’s a table predicting non-ferrous plate thicknesses based on current levels. These are not hard and fast rules and other factors such as width, span location etc. Engineering is more than calculations. The structural strength and suitability play hand in hand, plus installers playing a hand in correct actions on site. With the best will in the world if an installer mis-terminates a gland, does not earth correctly or similar then you have more headaches than eddy currents.
| Application | Current Range (A) | Typical Non-Ferrous Thickness (mm) |
|---|---|---|
| Light Commercial | <100A | 2 mm – 3 mm |
| Medium Commercial | 100A–400A | 3 mm – 5 mm |
| Industrial | 400A–1000A | 5 mm – 8 mm |
| Heavy Industrial | >1000A | 8 mm – 10 mm |
Practical Implementation Steps
- Identify Problem Areas:
Inspect the gland plate for overheating, hotspots, or signs of EMI affecting nearby equipment. Note the conductor sizes, hole diameters, and gland plate thickness. - Cut Channels (Ferrous Plates Only):
Using the recommended widths from the ferrous table, cut channels between the holes. Ensure the channels fully penetrate the plate to break the eddy current loops. - Monitor Results:
After implementation, monitor the gland plate’s temperature and check nearby equipment for reduced EMI. Confirm reduced heat and improved EMC performance.
Benefits of Using Non-Ferrous Plates
- No Eddy Currents:
Non-ferrous materials inherently resist eddy current formation, eliminating associated heat and EMC issues. - Lighter and Thinner:
Non-ferrous plates are often lighter and can be thinner while maintaining structural integrity. - Improved Efficiency and Safety:
Reduces energy loss and minimizes the risk of overheating and component failure.
Let us wrap this up.
Eddy currents in ferrous gland plates can cause significant operational and safety issues, including overheating, energy inefficiency, and EMC problems. While non-ferrous gland plates provide the best preventive solution, channel cutting remains a highly effective method for mitigating these effects in ferrous plates. By understanding the science behind skin depth, EMC, and appropriate material choices, you can ensure safer and more efficient electrical installations.
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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.
