Introduction to conductivity and mechanical performance in grounding joints
Trains have multiple electric subcomponents located on top or underneath passenger wagons which must be protected against short circuit events. If such events result in a break in the power supply they will cause severe damage to components or even injury to passengers or operational staff.
There are many standards relevant to electrical safety in trains to help avoid such issues, some of which are included below for reference.
DIN EN 50343 (EN 50343)
Rules for installation of cabling
DIN EN 61439-1 (IEC 61439-1)
Low-voltage switchgear and control-gear assemblies
DIN EN 61373 (IEC 61373)
Shock and vibration tests
DIN EN 60721-3-5 (IEC 60721-3-5)
Part 3: Classification of groups of environmental parameters and their severities
DIN EN 60068 (IEC 68-* or IEC 60068-*)
Environmental testing including tension, bending, salt mist, electrical resistance, sand and dust, vibration (see also IEC 61373), change of temperature, dry heat, sealing, shock (see also IEC 61373), torsion, torque.
To prevent short-circuit events the current from the power supply must be bypassed direct to ground via electrical bypass paths which require a joint to attach the wiring. The joint is critical in ensuring that the optimal level of conductivity is maintained under all possible conditions. As with any safety critical joint the installation process must be able to ensure joint integrity for every installation and the joint must perform over time.
Historically, whilst testing indicates that the correct levels of conductivity can certainly be reached using a grounding stud solution (which is typically used in these applications), the joint’s mechanical performance can sometimes be inferior. Usually this results from poor installation process which results in sub-optimal material contact is sufficient for the required conductivity. This presents a risk that the electrical bypass path is not fully functional during operation. The performance differences of a joint ‘on paper’ and in real life can be significant.
Therefore, the desired state of a grounding joint system should be to deliver:
- The required level of conductivity appropriate to the short circuit event risk.
- The mechanical performance required to guarantee the long-term performance of 1).
The three primary tangible system elements of a grounding joint are:
- Sheet material (material type, material thickness, contact area)
- Grounding cable configuration
- Fastener (diameter, material etc.)
Each of these play a role in achieving aims 1), 2) above.
There are several other intangible factors which play a role in every successful joint, which we will discuss in more detail in part 2 of this article. The first is reliable control and feedback of appropriate Installation parameters. The second is proof / documentation of each ‘passed’ grounding joint.
Howmet Fastening Systems (HFS) based in Kelkheim, Germany undertook significant testing, with support from International Institute for Product Safety in Bonn, to understand the relationship between mechanical performance and electrical conductivity in joints based on the criteria defined above. This study summarises the results of the testing.
For details of how we tested please refer to Appendix 1 (link).
Conductivity Performance (2.1)
The graph (Fig.*) shows the relationship between material, time, material thickness and current for size M6. For alternating current, the function is indicated as RMS on the y-axis.
Short circuit resistance: The shorter the duration and the higher the current is, the more the I peak, and the induced electrodynamic force becomes relevant. Vice versa, the longer the current flow is maintained the more increasing heat levels reach the critical value which is the base effect for the ability to withstand a short circuit. (>1 sec).
Testing was done using a Cam-Safe installation system to ensure a correctly installed fastener: the results do therefore not account for joint variability caused by inconsistent installation processes.
The summarized key findings of the testing are listed below:
• Thicker material performs better than thinner versions in all cases.
• After 1 second almost all materials were at the same level, due to the development of heat and the resistance against the electrodynamic force. The exception was 1.5mm stainless steel which is lower due to its natural properties.
• Shorter timescales are most critical to short circuit events. The timeframe 0.05 seconds to 0.10 seconds is most relevant, including in the context of rail applications.
• It is in the shorter timeframe that the differences between material types and thickness are most noticeable.
• Aluminum allows the highest current which decreases the longer the test duration.
• Corrosion resistant steel is lower in principle, but the difference decreases the longer the duration. This effect is due to electrodynamic forces which are better supported by the material strength of steel.
• Thicker material will offer greater conductivity. If conductivity is the priority, then aluminium sheet will offer the best performance. However, carbon steel only sacrifices minor conductivity for much greater mechanical performance in both 1.5mm and 4mm materials.
To explore the relationship between conductive performance and mechanical performance, we will examine the implications of our results more broadly below.
Contact Area (2.2)
Our testing highlighted that the crucial contact is established within the hole itself (whether drilled, stamped or cut) and not on the top or bottom of the fastened components.
This means the sheet material can have an external isolation layer, for example plating, protection foils or paint without negative conductivity implications; only the material within the hole must remain in its raw state. It is vital that the internal surface area is clean as oil and dirt can form a barrier and reduce contact. When using Aluminum, the bore hole must not be anodized, where necessary by re-drilling the hole after anodization to recreate the raw material state within the hole.
The mechanical performance by contrast is reliant on the fastener contact with the top and bottom of the material.
Material Thickness (2.3)
The tests showed that material thickness (in combination with hole size / fastener diameter) plays an important role in grounding joints, especially for AC applications. An installation into a thicker structure can, due to the larger contact surface and mass, support higher mechanical forces as well as dissipating greater levels of current and heat than a thinner structure over the same time span.
A thinner material will limit the conductivity and mechanical performance of the joint but will typically offer a lighter weight solution. Use of thinner material allows less room for error in case of a bad installation, with any loss of contact area have a larger relative impact on conductivity levels. Refer to our recommendations for thin sheet joints.
A smaller contact area = large electrical resistance = lower conductivity
A larger contact area = lower electrical resistance = higher conductivity
Material Type (2.4)
The three materials typically used for grounding joint structures are Aluminium, carbon steel and stainless steel. Aluminium is an excellent conductor of current and heat. Stainless steel can dissipate current and heat less well but provides much better mechanical strength. Carbon steel fits between the two; it has good mechanical properties whilst its thermal and electric conductivity are much better than stainless steel.
The table shows the relevant specifications for Aluminium, stainless and carbon steel. Copper is listed as a reference due to its relevance in electrical applications.
|Sheet metal material||Tensile strength||Yield strength||Conductivity IACS||Electric conductivity||Heat dissipation||Spec. electrical resistance|
|Aluminium||180 min.||80 min.||62||35,9||236 (Al99,5)||0,03|
|Carbon steel||290 min.||140 min.||18||10,5||40-60||0,10|
|Corrosion resistant steel||500 min.||200 min.||2||1,4||15||0,72|
The material should be chosen based on a combination of the desired levels of conductivity and mechanical performance of the joint.
Cable specifications (2.5)
The cross section of the cable must be specified by the customer to meet specific application requirements, including the choice of material, material thickness and fastener. If the ability to withstand short circuit is too low the resulting heat will be too high, with the possible result that the cable might burn through.
Each connecting point must be able to withstand the highest requirements defined in relevant regulations to ensure safety over the lifetime of the application. It is important that the chosen fastener delivers the mechanical performance required to secure the cable.
Mechanical Performance (2.6)
When selecting a fastener for grounding applications it is important to choose one that maximizes the surface contact within the hole (see 2.2 above).
The image below shows a cross section diagram of a correctly installed grounding stud.
One of the biggest concerns is the variability of contact due to inconsistent installations. A poor installation which does not achieve the desired surface contact will directly impact the level of conductivity the joint delivers. The ideal would be a system which enables 100% desired contact consistently through a guaranteed method, or at least identify a bad installation which can then be replaced.
Picture of a failed (sloped) installation
A sloped installation (showing a gap between contact ring and plate, see picture above) can result from poor installation angle and/or long installation times. In addition to negatively impacting the predictability of surface contact within the joint, it can also lead to mechanical underperformance (poor torque-out) and risk of complete failure. With appropriate tooling concepts, like the Cam-Safe system, this failure mode can be reduced significantly.
Once correctly installed it is important to ensure longevity. A fastener that offers a permanent joint is recommended. Fasteners such as the Cam-Safe ground studs (made of two components; a tension stud or nut and a contact-ring) utilize a force-controlled installation process to expand the fastener within the panel hole creating a permanent joint with 1) maximum surface contact, 2) resistance to vibration and tampering and 3) minimum electrical contact resistance.
When working with grounding joints in rail applications safety must always be the absolute priority. The results identify several important considerations in joint design: the specifications of each of the primary joint components is important in achieving balance between conductivity and the mechanical performance of the joint. It is clear that the conductivity and mechanical performance go hand in hand. Without a good installation and the correct contact area between fastener and parent material the joint conductivity performance is at risk.
In part 2 we will explore the benefits of the Cam-Safe system in achieving guaranteed successful installations and mechanical performance.
Click here to view Part 1 Appendix containing more details of testing methods that have been used.