Safety  working with 240V AC - Page 1
Forum section: electronics / beginners

author: cprrw132
author_rank: Newbie
timestamp: July 19, 2014, 08:49:45 am
content: G'day Group .I have a safety question .I have a earth leakage on my work shedthat i know works .This is no reason to be lax when working with 240V AC .My question is I have an isolation transformer that i have never used .Is it safer to just use the Earth leakage ,Or use a isolating transformer when dealing with live 240V AC ?John T ozz

author: IanB
author_rank: Super Contributor
timestamp: July 19, 2014, 08:57:50 am
content: It's not really clear what you plan to do with this AC mains voltage?In general, one does not work with live mains voltages. One disconnects power before working on the circuit, and then keeps ones hands clear of the circuit when reapplying power for testing.

author: mij59
author_rank: Frequent Contributor
timestamp: July 19, 2014, 09:21:59 am
content: A earth leakage circuit breaker only protects against current flowing to earth ( is it a 30 mA one ? ), so if you make a short, or touch line and neutral the breaker won't trip.Over current and earth leakage are the fist line of defense, you can't replace them with an isolation transformer.An isolation transformer can be use to make measurements with an earth referenced device like an oscilloscope.

author: Rerouter
author_rank: Super Contributor
timestamp: July 19, 2014, 09:30:27 am
content: There is almost nothing you can sit between you and mains that lets you recklessly play with it,An earth leakage breaker relies on a significant enough current flowing through you for a few cycles to trip, there are medical ones that use a much lower current, though they will trip on just about anything, for the obvious reasons 90% of the time making them impractical, in other words you still get zapped and risk death, you just do this for a shorter period of time than if it was not there,An isolation transformer can make things slightly more dangerous depending on what you are doing, for one thing the earth leakage breaker does not care if you grab both of the secondaries, and one that has been sitting in a garage for half a decade, especially an older model should be checked that there are no obvious shorts between the primary and secondary (The amount of times i have seen someone call an auto-transformer / variac an isolation transformer is laughable)Have a think about how to describe what you are trying to do and i will be more than happy to help., and like the above poster, the general consensus is to put your measuring points in place and then power on the device, when people go poking around powered on electronics they should either know the risks well, or in the more likely case the circuit is powered by the secondary of a step-down transformer which does not expose any lethal voltages,

author: cprrw132
author_rank: Newbie
timestamp: July 22, 2014, 03:05:27 am
content: Thank you for all your responsesTo Ian B.Yes my opening question may appear a bit vague .Your safety concerns truly noted.I was looking for opinions of running a Isolation Transformer after a 30mA earth leakage unit.Thank you to mij59You have answered my question the same one I had answered my self but was looking for some other inputs.In response to Rerouter .Spot on reply .the isolation transformer I have was new last year,www.tortech.com.auP/n: ISO-600ES 240V AC 600VAthis transformer will rest on a shelf until I have a need to decouple two circuits or a good reason to isolate a ground potential .I appreciate your offer of further assistanceJohn T .

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 22, 2014, 05:22:59 am
content: HiI have the same Isolation transformer. I am guessing you are from Australia.I use it often with and RCD connected upstream and sometimes with a variac downstream.You probably have an RCD in your shed not an ELCB. They are different.I am not sure of the situation where a working Isolation transformer added to a circuit would make a circuit more dangerous. This has never made sense to me, please give an example if you can.One nice thing about a small isolation transformer is that you get an audible indication when as the current approaches maximum.You have an extra fuse, you have a certain amount of current limiting otherwise.IMO an isolation transformer is also a must have if you are using a variac.

author: diyinhk
author_rank: Contributor
timestamp: July 22, 2014, 05:47:21 am
content: is it possible to put human body at ac240 potential level and work with ac240 is more safecaution: anything related to the main is danger and should be extreme care or hazard to life

author: Kremmen
author_rank: Super Contributor
timestamp: July 22, 2014, 06:03:29 am
content: Quote from: diyinhk on July 22, 2014, 05:47:21 amWhat you are effectively asking (if i understand you correctly) is, if it is safer to be at the potential of the live wire insead of being at the potential of the ground (these being the 2 points between which the 240 VAC voltage can be measured. Bear in mind that all _voltages_ are relative between 2 potentials.The answer is: yes, it is possible and no, it is not more safe. In fact it is far, far more dangerous since now every object at ground potential is a shock hazard. The circuit is closed when there is a conductive path from live to ground and it matters not at all where your body is initially located in the circuit.I have seen these HV linemen doing repairs on the 400 kV lines between the high pylons. They use helicopters to fly close, then equalize charge using a long lance probe after which the daredevil repairman attaches a monkey swing to the line and jumps across to do the repairs. In that case there is zero chance of accidentally touching ground so there it makes sense to be at the line potential. Not so in a domestic environment.

author: G7PSK
author_rank: Super Contributor
timestamp: July 22, 2014, 07:41:38 am
content: As soon as you clip your scope test leads to a DUT that is supplied by an isolation transformer that device is no longer isolated from earth therefore if you touch anything live on it you will still get a shock lethal or not.I have seen live or hot repairs carried out to 415/240 power cables in trenches with 50mm of water in the bottom and the linesman standing on a rubber mat and wearing rubber gloves and in one case just the mat and no gloves.

author: SL4P
author_rank: Super Contributor
timestamp: July 22, 2014, 10:58:44 am
content: Question:I'm not a mains fiddler, but curious what the difference is between an RCD and ELCB.As I understand - both wind the Active and Neutral around a toroid, and onward to the load.A 'sense' winding detects any current imbalance (e.g. >30mA) between the AN circuits - with the assumption it's going somewhereelse(i.e. ground) - and trips the contactor/relay to isolate the load from the mains.Isn't that whatboththe RCD and ELCB do ??

author: Rerouter
author_rank: Super Contributor
timestamp: July 22, 2014, 11:11:46 am
content: From poking around it appears that an earth leakage breaker like the name suggests, monitors only the return earth wire currents, intended to trip when a live to earth fault occurs inside a device, but ignores say a person barefoot on concrete touching live, this is where an RCD comes in, and covers against both these events,

author: SL4P
author_rank: Super Contributor
timestamp: July 22, 2014, 11:37:03 am
content: Quote from: Rerouter on July 22, 2014, 11:11:46 amHmmm ok. Sounds like a piece of almost useless safety gear - won't work with double-insulated or other non-grounded equipment... ?!I think I'll still use the terms interchangeably for want of clear definition.When the term RCD became popular (I guess mid 80's) - my thought at the time was that it is non-technical for the sheeple out there that can't understand or remember four mildly technical words in a row (or in some cases, four letters!).

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 22, 2014, 12:10:35 pm
content: QuoteI dont think I've ever actually seen an ELCB, probably because the laws have mandated RCDs instead of ELCBs for probably 20 years.I guess they are not used anymore because they are no where near as safe as RCDs.ELCBs from memory also had a lot of trouble with false tripping.And at least with the term RCD, people can look it up and it is clear what device you are talking about, although it is an easily forget-able name.

author: SL4P
author_rank: Super Contributor
timestamp: July 22, 2014, 12:16:03 pm
content: I should have done this earlier -- WIKIPEDIA knows all !http://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/ELCBInteresting history