This article started with a simple question: what is the correct washer to use to secure a ring crimp terminal on a threaded bolt?
I have seen either spring/split washers or internal/external star washers being used.
I asked on LinkedIn and found some good advice, some received wisdom and “we’ve always done it this way”, but not much in the way of citeable standards or references from technical authorities.
Goals of Earthing / Bonding System
The fundamental goals of the electrical fixings in the protective earthing / bonding system are:
- Provide (at assembly and maintain during use) a low resistance, potentially high current (tens/hundreds of Amps depending on supply) electrical contact
- Not corrode or loosen under the normal environmental operating conditions to the point where the resistance goes out of specification
In reality there are many factors that one could worry about:
Common Safety Standard Review
A review of the more frequently used safety standards for electronic products yields the following clauses.
|“parts fixed by means of screws or nuts provided with self-locking washers or other means of locking are not liable to become loose or detached
NOTE Spring washers and the like can provide satisfactory locking”
|“Screw connections shall be secured against loosening”
|“The contact pressure required for a bonding connection shall not be capable of being reduced by deformation of materials forming part of the connection”
|“Screws and nuts that make a mechanical connection between different parts of the appliance shall be secured against loosening if they also make electrical connections or connections providing earthing continuity. This requirement does not apply to screws in the earthing circuit if at least two screws are used for the connection or if an alternative earthing circuit is provided.
NOTE 1 Spring washers, lock washers and crown type locks as part of the screw head are means that may provide satisfactory security.”
|“27.2 The clamping means of earthing terminals shall be adequately secured against accidental loosening.”
|Clamping means of earthing terminals for external conductors shall be adequately locked against accidental loosening.
|“parts fixed by screws or nuts provided with a locking washer are regarded as not liable to become loose”
(note clause not specifically related to earthing)
Typical Locking Fixings
The below image shows a variety of locking methods that I would consider acceptable for this purpose.
The standards are not prescriptive about the type of locking washer to be used.
Spring washers, lock washers and threaded fastener locking features are all valid approaches.
No washer is also an acceptable method provided there is a locking nut of some kind of suitable locking adhesive used.
Two independent fixings are considered to be acceptable in some standards.
Testing Testing Testing
In all cases, conformity with the standard is checked by inspection and/or appropriate testing. Testing is key.
Testing the protective earthing / bonding system includes measuring the resistance and/or measuring the current handling capability of the connections.
If you are the manufacturer and wanting to use a non standard fixing method then it may be acceptable. Any non-standard or atypical methods would need adding to the product compliance risk assessment.
The testing specified in the standard is the bare minimum and additional testing may be required to demonstrate that everything is indeed safe. Testing could include extended high humidity environmental testing to check for corrosion and representative vibration testing to make sure that loosening does not occur in use.
Selecting suitable environmental test levels for your product can be based on your experience as the manufacturer with typical operating environments, or perhaps using the ETSI EN 300 019 environmental engineering standards.
Of course, the simplest way is to just use a standard washer to reduce arguments.
Like all simple questions, there is a surprising amount of depth and possible considerations, including:
- Corrosion, plating, passivation, surface oxide layers, dissimilar metals. This is a book in of itself!
- Considering the current path. Using a locking washer with a small surface contact area in the primary current path can increase the resistance. Aiming for a larger surface area with a good quality connection would be optimum
- Surface preparation: clearing paint, anodising, rust, or oxidation.
- Minimum fixing size. Some standards call up a minimum 4mm diameter and number of threads engaged for certain types of screw fixings. This is not universal across all standards. If in doubt, selecting all threaded hardware to be at least M4 in diameter seems like a sensible option.
I would be very interested to hear of studies, standards, procedures, reports… indeed any published material that covers this topic of washers and fasteners specifically for electrical connection.
References & Links
- My original question posted on LinkedIn asking about washers
- NASA fastener design manual, page 10 has details of locking mechanisms
- The always amusing and informative AvE
- NordLock brand washers under the Junker test