Protective grounding

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.

There are lots of capacitive and inductive coupling to ground. The isolation is not perfect.

How does an aircraft sees (or feels) a lighting strike? You must have seen the lighting discharge electrodes (what are they called?) glowing up at night?

Only a few mA is sufficient to kill an adult human. Few microamps are sufficient to stall the heart. Once the current enters the body, it picks up the veins and arteries (that have lots of ions and high conductivity) and reaches the heart quickly. Most of the voltage drops take place over the epidermis.

I am no expert only learning and trying to understand. What in case a tree touches those wires or one of the wire fell to the ground. Will then any current be carriing trough ground?

And sub question, why TN grounding system (residental block buildings) don't use RCD? I understand that having a common ground with the transformer then in case of a short circuit great current will be drawn since resistance is minimum and circuit braker could interupt quickly (4 msec). But what if someone touches the live wire? Then due to body resistance current for the braker to interupt would be unsuficient and person would be electrocuted.

I have seen tree branches touching live high voltage wires along the road. Sparks fly.
I saw a tree with a branch burned away due to touching high voltage wires.

Our neighbor felled a tree against a high voltage wire at the end of our driveway. A complex pattern of sparks jumped around at the place of contact, setting the bark on fire, and eventually eating through the wire whereupon the tree fell to the ground.

The usual long distance very high voltage transmission lines only run three wires, with the transformers at each end connected in delta. There is no neutral wire.

The reason for this is it can transmit the most power, with the lowest losses, and with the least amount of copper.

There will definitely be a ground fault interrupt system that will completely disconnect power if one power line becomes suddenly grounded due to some serious fault. But the threshold trip current will be set fairly high to prevent nuisance tripping.

The system is not completely floating, as there will be considerable capacitance to ground, especially around the transmission towers, plus some leakage current to ground at every dirty cracked dust covered insulator.

So the system sort of ends up having a very poorly balanced voltages around ground potential. Any direct physical path to ground from one phase will conduct current to ground through the stray coupling of the other two phases to ground.

So it can kill you for sure, even though it is in theory a fully floating system.

svizoman said:

What in case a tree touches those wires or one of the wire fell to the ground. Will then any current be carriing trough ground.

Click to expand...

In fact, all the transmission towers carry one thin wire (the topmost one) that is at a ground potential. If a wire breaks and touches the ground, a phase to ground fault is signaled and the location of the break is determined from the resistive network using a principle very similar to Wheatstone bridge (I have forgotten the details). The contact is not perfect and the fault current is not large (the power line towers carry 100s of A each phase) but the imbalance is easily detected.

The imbalance current goes across the transformer (across the windings) but I cannot tell (because I do not know) where exactly the current leaves the ground again to complete the circuit.

in prctise electricity is wery complex, I really do not understand now how this works but no matter. I have still some question regarding IT grounding.
In shematics there are 4 wires (L1, L2, L3, N), but under description they say there's no neutral and under schematics phases that go trough consumers are connected together and end. I don't understand how a complete circuit is made when a single phase consumer let's say bulb is used.

https://www.google.si/imgres?imgurl...vjPAhXD6xQKHc_gB4IQMwgkKAUwBQ&iact=mrc&uact=8

If you connect a bulb between the line (220V) and the neutral, it will light- the circuit is therefore complete. If you now connect the bulb between the line and the local earth (the third pin) it will still light- that suggests that there is some way for the circuit to complete. The neutral is locally isolated (it is not connected to ground at the consumer end) and a voltage measured between the neutral and ground is usually small. In a three phase star connection, the neutral voltage should be equal to the sum of the three voltages on the three phases and with a balanced load is close to zero. The neutral is actually connected to the ground at the generator end (so that all the three phase voltages are referenced with respect to the ground) and the neutral at the consumer end is derived from the local distribution transformer and is not connected to the ground.