How to judge the result of Cord Bending Test?

Roland chung

Trusted Information Resource
#1
Hi all,

We now have a new portable device which use non-detachable power supply cord. The test for the connection of power supply cord (including cord anchorage and cord bending test) is therefore needed to be performed.

I am new for above mentioned tests, since our equipments almost use IEC 60320 appliance inlet. That means cord anchorage and cord bending test can be waived.

For cord anchorage test, it is very clear. But for the cord bending test, I am confused. The standard states that immediately after the mass has been attached, the curvature of the cord shall nowhere be less than 1.5 D, being checked by a cylindrical rod with a diameter of 1.5 D.

As I know, the curvature is the smaller the better. But the standard requires the curvature shall be more than 1.5 D. Is this a mistake of the standard? I would think the curvature shall be replaced by radius of curvature. Because of the curvature is the reciprocal of the radius of curvature.

Also, I can not understand very well why the test is checked by a cylindrical rod with a diameter of 1.5 D. What the rationale is?

Furthermore, is a MAINS TERMINAL DEVICES must in this case? Can I solder the power cord directly onto the PCB or other components (e.g. switch/ lampholder)? If yes, whether additional fixed methods are required?

I am sorry for several questions. I did not have experience in the connection of power cord.
 
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somashekar

Staff member
Super Moderator
#2
There is no mistake in the standard. With the load suspended, the cord will have to be subjected to bends at 90° in each direction over 2 smooth pins that are 1.5 times the D of the cord.
All the individual wires in the cord will be in a circuit to indicate continuity. The equipment that performs the bending will have a counter and will stop at the preset number of bends completed. However if any of the wires in the cord snap, then the machine will stop, indicating which wire has snapped and after how many bending cycles.
 
Last edited:

Roland chung

Trusted Information Resource
#3
Hi Somashekar,

Thank you fo your input. But I am afraid we are not talking about the same thing.

For bending test, you have two options. One is flexing test which specified in IEC 60335-1, the other is 1.5 D dismensional test. I think the description you made is from IEC 60335-1 which as an alternative test if the equipment fails the dismensional test (In 2nd edition, the standard requires to perform 1.5 D dismensional test first. However, this requirement has been changed in 3rd edition).
 
R

raymond02302

#4
hello roland,
The rational why the standard set up the requirement for cord guard is to ensure that the supply cord is not bent too sharply, it protects the insulation of the supply cord from abration or scrab.
I think the standard use the wrong word. As our common sense, the small the cuvature the less sharply the supply cord is bent.So the requrement "culvature more than 1.5D" is reversed to the rational. I guess the standard is intended to use the word "Radius of curvature ",this parameter is inverse proportion to the culvature. Therefore, it can be used to accurately express the requirement"more than 1.5 D" and comply with the rational.
You can't solder the power supply cord onto the PCB withou other securing means. The conductor should be hooked in before soldered onto the PCB, furthermore, if there is no coupler used, cord anchorage and cord guard are required.


Raymond
 

somashekar

Staff member
Super Moderator
#5
You can't solder the power supply cord onto the PCB withou other securing means. The conductor should be hooked in before soldered onto the PCB, furthermore, if there is no coupler used, cord anchorage and cord guard are required.
A very good point from raymond. A solder joint can never be taken for providing the mechanical strength. It provides electrical joint. Other reinforcement types as suggested must be in place.
 

Peter Selvey

Staff member
Super Moderator
#6
Just to confirm the first point: the radius of the curvature shall be greater than 1.5D. This requirement is normally easily met if a good quality cord guard is used that limits the bending (in fact, I never saw a case where the flexing test was needed).

For the connection, in principle soldering is OK. Retention of conductors is a separate issue that would be needed regardless the type of connection (e.g. screw, plug, soldering etc). See 8.10.2.

However, a special concept for medical equipment is that the power supply cord should be easily replaced if if damaged. You can see this in several clauses, in particular 8.11.4.5.

I guess the thinking in the standard is that in the rough medical environment, cords are often damaged and should be able to be quickly replaced without have to send out for service (or throwing away).

So, from this point of view a simple terminal block marked with L / N / PE symbol, located in an easily accessible place, easy to inspect is the best option. Soldering direct to components increases the risk of a wrong connection during cord replacement.

If the equipment is small, home use, inexpensive, there might be justification (e.g. risk management) to follow IEC 60335 which allows "Type Z" cord attachments which are intended to be thrown away if the cord is damaged. In this case, the screws etc must be special types to prevent easy dismantling (all this is from memory, so need to check 335 for details).
 

Roland chung

Trusted Information Resource
#7
Thanks, peter. Your input is very helpful.

In addition, could you tell me how to distinguish the cord anchorages, cord guards and strain relief means? Can they be the same?
 

Peter Selvey

Staff member
Super Moderator
#8
Cord anchorage and the cord guard are separate issues, but it is possible to buy a single component which does both functions. For example, cord guards with a gripping function on circular cords.

Strain relief is provided mainly by the cord guard. But, during tests the cord anchorage and the enclosure will usually move a small amount, so in addition you need a bit of slack to the terminals to allow for this movement. This is also part of "strain relief".

As per 8.11.3.5 e), the earth conductor should be a bit longer than the phase conductors, so that in the event of complete failure of the cord anchorage, the earth is the last to be subject to strain.
 

Roland chung

Trusted Information Resource
#9
Strain relief is provided mainly by the cord guard. But, during tests the cord anchorage and the enclosure will usually move a small amount, so in addition you need a bit of slack to the terminals to allow for this movement. This is also part of "strain relief".
For the words in red, I have a little bit of confusion. The standard actually requires the power cord to be disconnected from the mains terminals before performing the test. So the slack for the terminal seems meaningless.
 

Peter Selvey

Staff member
Super Moderator
#10
I think the point of disconnecting is so that you can see if the wires have moved. Movement more than 1mm at the terminals is a failure (2mm at the cord anchorage).
 
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