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Surge protection for 24 VDC power output cable

Peter Gregson
Prodigy 210 points
Other Parts Discussed in Thread: TVS3300, TVS2700

Hello,

I am designing a system that requires the master module to supply 24 VDC at 3 amps to a remote module that is located in another building via a 60-metre long, single, twisted-pair (power and ground) cable.  The power supply in the master module is limited to 3 amps and so this is a power-constrained application.  However, the cable must be routed outside.  The cable used for supplying power will be in a bundle with a Modbus cable.  The system must comply with standard  IEC/EN 61000-4-5 at Criteria A or B. My questions are as follows:

1.   Does this application require the system to be tested using a combinational wave generator with 2 ohms of source resistance?  If not, what would be the source resistance used for testing?  I ask because selection of the protection devices is hugely impacted by the level and source resistance used for the test.  Perhaps I missed the relevant section, but I found IEC/EN 61000-4-5 to be unclear as to whether:

i.   this should be considered a direct connection to a low-power source, in part because the cable goes outside, in which case the test would require a source resistance of 2 ohms ; or

ii.  it is a connection to a low-power source between power and ground, in which case the source resistance would be 12 ohms; or

iii. it is a connection to a data cable (clearly it isn't data, but there are many industrial signalling circuits require 3 amps at 24 volts) in which case the resistance would be 42 ohms.

2.   Is there an advantage to shielding the power cable with the shield connected to the enclosures at both ends? If so, won't this lead to ground loops together with the protective earth?

3.   Is there a reference design available for this application?

4.   What devices would I use to provide adequate protection?  The TVS3300 family or TVS2700 family could handle scenario 1.iii above (Ipp of about 24 amps), but at lower source resistances the value of Ipp gets very large, very fast, up to 2kA at 4kV with a 2-ohm source resistance.

Can anyone provide some clarity and/or assistance, please?

Thank you,

Peter

  • 0
    TI__Genius 12410 points

    Hello,

    I believe you would fall under the 42 category because they only way things can surge into your cable is capacitive coupling.

    Adequate protection is up to you for what level you want to pass. Our devices can do 1kV surge at 42ohms but if you would like more you would need to look at typical TVS devices or varistors.

  • 0 in reply to Cameron Phillips
    Prodigy 210 points

    Hello Cameron,

    Thank you for your very prompt response.  It is much appreciated.  

    I am pleased to hear your answer.  That is what I thought (that the 42-ohm scenario is correct) but that doesn't say that the test labs would agree with me.  So I thought that I would ask the folks on the forum.

    This resolves my question.  I will hit the green button now.

    Best regards, and thank you,

    Peter