DC - high current and heat

Discussion in 'Top Tips & Tricks' started by Sidney, Apr 11, 2014.

  1. Sidney

    Sidney Funster

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    If like me you run lots of kit via an inverter it's worth checking that you have good contacts on all the electrical connections. Last year a fuse holder began to deform and melt causing me some angst on a hot day!

    When replacing this is fuse block I tinned the end of the multiple strands on the thick cables to ensure a good contact.

    Today I was checking there's no overheating!
     

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  2. Techno

    Techno Funster Life Member

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    I crimp a tube lug on the end then cut off all but the crimp, this makes a really solid object for screw terminals to bite on.
     
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  3. Fireman Sam

    Fireman Sam Funster

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    You’d be horrified by the number of fires caused by resistance heating (loose or bad connections). Good, helpful post Sidney.
     
  4. scotjimland

    scotjimland Funster Life Member

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    Sorry Techno.. but If I am understanding this method correctly.. I have to disagree.

    Modern terminal blocks don't have screws biting on the strands, rather the strands are compressed between two small plates..

    A solid copper sleeve does not make a good connection.. the plates cannot compress it.

    I worked offshore on oil and gas platforms as an instrument tech, this would not be allowed, nor would soldering the stands together ..same effect.

    Even if it's the bog standard chocolate block connector the same principal applies. Soft strands make a far better connection than a solid piece of copper.
     
  5. Techno

    Techno Funster Life Member

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    DC fuse holders either have a stud (preferable) or a grub screw terminal. This is not Merlin Gerin :Laughing:
    The flat face of a Hexagonal crimp is perfect for a large grub screw
    BTW instrumentation does not carry huge currents which is the point of this topic
    Example
    [​IMG]
     
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  6. scotjimland

    scotjimland Funster Life Member

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    a stud is not what you were describing in your first post ..
    and a screw terminal shouldn't have a solid copper lug in it ..
     
  7. Techno

    Techno Funster Life Member

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    This is the much better option
    [​IMG]
    [​IMG]
     
    Last edited: Apr 11, 2014
  8. pappajohn

    pappajohn Funster Life Member

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    in an industrial installation (i know this isnt industrial) we ALWAYS terminate stranded wire with either a soft copper or aluminium furel if it enters any kind of screw terminal equipment/plant.
    Domestic, we simply twist and double back then terminate
     
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  9. Techno

    Techno Funster Life Member

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    Grub screw terminals on fuse holders do not have metal plates to protect the copper
     
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  10. pappajohn

    pappajohn Funster Life Member

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    and the vast majority of 'stand alone' choc block connectors dont either.
    Usually only 'fixed at manufacture' items have clamp plates.
     
  11. scotjimland

    scotjimland Funster Life Member

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    two schools of though on this John.. some say ferrules maker a neater and better connection.. others say they are more prone to vibrating loose..

    off shore and in the Shell oil refinery.. they were banned.. yet in the SSEB power station I worked on they were mandatory :Doh:

    Only reason I can see is.. one is high risk(refinery) .. other is not.. (power station)

    this is a similar argument as the PTFE tape .. banned offshore .. yet ok in most other places..
     
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  12. Techno

    Techno Funster Life Member

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    And no good for DC high currents
     
  13. SMB

    SMB Funster

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    I have no idea what any of you are talking about :Rofl1:
     
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  14. TheBig1

    TheBig1 Funster Life Member

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    soft crimp ferrules are mandatory for MOD installations, or at least they were a few years back. even then there were discussions about whether they were mechanically safe in hazardous environments. vibration or constant snatching and pulling on connections were prone to failure

    in motorhomes, its hardly a hazardous environment and other than charging circuits and inverters they tend to be low current connections. some installations are dire, i agree, but going too OTT is pointless and adds expense and complexity where its not needed
     
  15. Techno

    Techno Funster Life Member

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    I'm now in a position to supply explanatory pictures.
    Clearly if welding cable was put into a screw terminal it would simply part down the middle and crush under a grub screw. So for fuse holders
    [​IMG]
    [​IMG]
    No arguments make your own choices :thumb:
     
  16. timdownieuk

    timdownieuk Funster

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    From what I've read (and experienced), tinning is actually a bad idea as it converts the wire end into something that's relatively soft and "plastic" with no elastic properties. Repeated heating and cooling cycles due to current load lead to a gradual loss of contact pressure that accelerates the heating due to falling contact pressure.

    I'm no electrician but I've heard this from several sources and it makes sense to me. I think you need to recheck the temperature periodically to check that it isn't overheating.

    Tim
     
  17. scotjimland

    scotjimland Funster Life Member

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    Yes indeed I will.. and I won't be using a solid copper end in a screw terminal block..

    your first picture clearly shows the screw biting into the copper.. a small moon shaped crescent.. very poor contact area and liable to overheat.. and vibrate loose.. your choice.. :Wink:
     
  18. Techno

    Techno Funster Life Member

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    That's not the ONLY point of contact, the whole lug is pushed hard against the back of the receptacle :Doh:

    BTW I've just removed that after nearly a year and NO it did not get the slightest bit loose nor did it get hot.
     
    Last edited: Jun 5, 2014
  19. Ed Excel

    Ed Excel Funster

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    Perhaps you could take a lead from electrical accessory manufacturers? Socket outlets have grub screw terminals, for termination of general wiring cable. Connection units have grub screws, for supply general wiring cable and compression clamps for load flexible cable.

    Also, as electrical designers we would never use single strand cables in grub screw terminals, because the screws are intended to cut into the conductor.

    All of which makes me wonder if using flexible cable for accessory wiring in a motorhome is the best advice.
     
    Last edited: Jun 5, 2014
  20. jezport

    jezport Funster

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    Re soldering the end that goes into the block. I ran a business refurbishing TVs etc a few years back and one of the causes of blown power supplies was an arching mains plug and 9 out of 10 times it was a soldered ends that had flattened over time and become loose. So dont solder the tails.
     
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