Author Topic: Why are mains earth and ground connected together on osciloscopes?  (Read 834 times)

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Offline little_carlos

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Hi,  why in scopes ( and maybe some other devices) ground and mains earth connected together? i make this question mainly because i saw dave jones video on how not to blow a scope, and i found that this is more like a risky thing than a safety thing, plus, i dont know the main reason of mains earth completely, can someone explain me?
 

Offline Delta

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Re: Why are mains earth and ground connected together on osciloscopes?
« Reply #1 on: March 30, 2016, 02:39:46 am »
Because mains earth IS safety earth / ground!
 

Offline nomadd

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Re: Why are mains earth and ground connected together on osciloscopes?
« Reply #2 on: March 30, 2016, 03:34:32 am »
..i dont know the main reason of mains earth completely, can someone explain me?

The "live" side of the mains supply is at a different electrical potential (think voltage) to the Earth (and that's Earth, the real Earth, and not simply some local "ground" inside a device.) So the mains electricity supply will readily flow to Earth, if given the chance.

Now imagine a device malfunctions and becomes "live"... You, the operator, could easily form a conducting path back to Earth, which is not a good thing for your health. :)

So, by actually "earthing" the device - that is, having a nice low resistance (pedantic: impedance) path back to earth through a separate run of cable - the "live" flow of electricity will take that route rather than you (and of course blow fuses, trip circuit-breakers, etc. along the way, hence shutting off the supply.)
 

Offline zal42

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Re: Why are mains earth and ground connected together on osciloscopes?
« Reply #3 on: March 30, 2016, 03:42:08 am »
What nomadd said. I can only add the observation that I would much prefer to burn out my scope than my body.
 

Offline BobsURuncle

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Re: Why are mains earth and ground connected together on osciloscopes?
« Reply #4 on: March 30, 2016, 03:53:20 am »
Suppose the common inside the scope is not grounded.  You stick your scope probe "ground clip" on a 300V rail in the device you are testing.  Now the scope common bus is 300V.  So you then go to connect a probe cable to the other BNC jack on your scope, the outside of which is scope common - and now 300V.  Your thumb brushes the jack as you connect the cable and a second later you are laying on the floor with the magic smoke escaping your ears.


 


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