Basic few questions

[basejumper31]

*a bit* like the truck analogy - I realise it's not exactly like that

 

[Richard Burns]

If you consider the human body as a resistor ( generally of approx 1000Ω in dry conditions) then the current that would flow through the body would be dependent on voltage in accordance with I = V/R.
If you have 230 V then approximately 0.23 A or 230 mA would flow through a human body, if the voltage were 55 V then approximately 0.55 A or 55mA would flow, in both cases this is potentially fatal. (This is where RCD protection to 30mA comes in, to protect a person from a fatal current)
However you can see that the current is dependent on voltage but it is the current that can cause the physiological effects such as heart arrhythmia, muscle contractions and death.
Because it is the current that is the key point this is the item referred to.

I should point out that the 1000Ω is only an approximation and if the body is wet then the resistance reduces considerably, which is why there are additional requirements for locations where the body may be wet or immersed in water.

 

[basejumper31]

Good morning (well, noon low lol) I am continuing my way through a Trevor Linsley text book ahead of the start of my college course 2365 lvl 2.

Linsley includes 'check your understanding' sections after each chapter. I'm completing the section after chapter one. Most of the questions he has provided answers for in the rear of the book. But for some he just says "the answers are to be found in the text of the chapter itself". Well I have checked the text of the chapter thoroughly, and I can't find a specific answer to this question, so I thought I'd post it on here and just see if anyone might be able/willing to offer any useful comments about my answer and how adequate it is.

p.59, Question 43:

Use bullet points to list a step-by-step safe electrical isolation procedure for isolating a circuit in a three-phase distribution fuse board.

My answer:

● Test the circuit with a voltage indicator to see that it is live
● Isolate it (i.e. disconnect it from all power sources)
● Test the circuit again to ensure that it is dead (no current)
● Connect the voltage indicator to a proving unit to ensure that it is not faulty.
● Secure the isolation (i.e. Remove all relevant fuses and use padlocks if necessary/appropriate)
● Place warning signs as needed to indicate work is being done
● Commence work

Linsley does state these steps in the chapter, it's just he doesn't say anything about three-phase, or fuse boxes in particular (well, he does in respect of the fact that they can sometimes have asbestos in them, but that's about it).

I'm just wondering if there's anything extra which needs to be done in terms of safe isolation procedure when it comes to three-phase systems as opposed to single phase.

Anyway thanks for reading, I hope everyone's had a good week and looking forward to the weekend!

 

[Richard Burns]

Taking a full approach to safe isolation
Correctly identify the supply you wish to isolate.
Seek permission to isolate the power from the responsible person so that you do not present a danger from unexpected loss of power.
Isolate the supply.
Lock off the supply and retain the key with you.
Post warning notices on the locked off supply.
Prove the voltage indicator is working on a known live source or proving unit.
Test using an approved voltage indicator to GS38 to ensure that all conductors are dead (L1/L2, L1/L3, L2/L3, L1/N, L1/E, L2/N, L2/E, L3/N, L3/E, N/E).
Prove the voltage indicator is working on a known live source or proving unit.
Consider the risks of adjacent live parts.
(issue a permit to work, if required)
Start work on the dead circuit.

 

[Former Sponsor TM]

1) Identify circuit or supply needing isolation;
2) Ask client if OK to isolate;
4) Lock off the isolation with an appropriate lock (keep key on your person or locked away)
5) Put up a warning sign and a barrier if needed warning others that you are working,
5) Check your voltage indicator on a known voltage supply (approved proving unit or incoming supply)
6) Check between: L1 - CPC, L2 - CPC, L3 - CPC, L1 - N, L2 - N, L3 - N, N - CPC, L1 - L2, L1 - L3, L2 - L3
7) Check voltage tester again a known voltage supply
8) Commence work