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Protective grounding

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merk

New Member
First my appologize this topic thus not entirely belong here but yet please help...

In IT grounding system transformer is not grounded and thus there is no need for RCD device since touching live wire does not complete circuit. Instead ground fault detector is installed that measures resistance from all 3 live wires compared to the ground. If resistance is lower than 50k ohms alarm is trigered.
My question is why do you get electrocuted touching wire of high voltage transmision lines when power plant transformer coils also isnt grounded and no current flows through ground.
 

spec

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Hi merk,

You don't say where you are from so we have no idea what the electrical standards are in your local, but to answer your question, no if you touched one wire of an isolated supply you will not get a shock due to the voltage between the two conductors of the supply, but you may still get a shock from the static charge on the isolated supply circuit.

Also, with high voltage (110KV for example) AC supplies capacitive coupling may cause a shock.

But, my understanding, and the experts may correct me here, is that, for safety reasons, no factory, or domestic supply is isolated from ground. With a three phase supply the neutral point is normally connected to earth somewhere.

spec
 

Nigel Goodwin

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But, my understanding, and the experts may correct me here, is that, for safety reasons, no factory, or domestic supply is isolated from ground.
I would partially agree, in that supplies are normally grounded - but I would disagree that it's particularly for 'safety reasons' :D

Grounding one side of the supply (the 'neutral') offers some safety advantages under some circumstances, but causes just as many dangerous disadvantages under other circumstances.
 

spec

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I would partially agree, in that supplies are normally grounded - but I would disagree that it's particularly for 'safety reasons' :D

Grounding one side of the supply (the 'neutral') offers some safety advantages under some circumstances, but causes just as many dangerous disadvantages under other circumstances.
Yes, agree :)

But the safety regulations, as far as I know, mandate grounding for safety reasons.

spec
 

KeepItSimpleStupid

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Something is WAY misunderstood by the OP/TS. RCD's don't measure resistance. They take the normal path of current and the normal return path of current and subtract them with a "current" transformer just by winding the paths in opposite directions. There is a result winding that gives you the difference.

RCD's are usually a uk term. GFCI's are a US term. For one of these to work, I'd like to say a ground is not necessary, but that would be partially incorrect.
In the US we have Line and Neutral 2-wire outlets that can be grandfathered. The new standard is a 3 wire outlet with L1, N and Ground.
A 3-wire GFCI can be used on a 2-wire outlet provided the outlet is labeled (No Earth ground.

The system connects neutral, ground to earth at the point of entry. The transformer is a little harder to grasp. The primary is fed directly from ground and a single high voltage. The secondary is a center-tapped transformer and the center tap is connected at the house point of entry where it is earthed along with neutral. The main power entry box is where the system gets earthed, neutral connects to ground AND water and gas pipes get grounded as well for those with copper and iron pipe.

The system is fed with single phase and results in split-phase or a 120-0-120 type of system which is referred to as single phase.

At no other place will ground and neutral be connected together in a single dwelling. Any sub-panels will keep ground and neutral separate. I won't get into detached structures fed by the main panel.

It's rare for a utility to allow 3-phase for residential.

So, there are inadvertent paths to ground for non-double insulated stuff. A leaky toaster and a sink with grounded plumbing and a human touching the plugged in toaster and the sink faucet.

In this system ground is supposed to be a reference and carry faults. It's flawed because the outlet grounds of a circuit are daisy chained. A large fault at the end of the string will raise the ground point of everything downstream. Having each outlet a home run would fix this.

We have orange receptacles (commercial usually) that are called "independent ground" that have two grounds. Protective and reference.

Hospitals and RF transmitters may be designed with both grounds from the ground up.
 

dr pepper

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'grandafthered', another term to remember.
I found out the hard way about 'neutral borrowing' in my last house, when a empty fuseholder was live on both the supply and load contacts, much to my surprise.
 

JimB

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Something is WAY misunderstood by the OP/TS. RCD's don't measure resistance.
True.

BUT, have a look at these units made by Bender...


No, not him!

This company:
http://www.bender-uk.com/products/c...ult_location__127/isometerr_irdh575__192.html
In the above link we see the words:
The ISOMETER® of the IRDH575 series monitors the insulation resistance of unearthed power supplies (IT systems).
The equipment which I worked on in my day job used these things extensively and measured the earth leakage resistance of undersea cable.

JimB
 
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