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Old 16th Oct 2018, 12:22 pm   #1
suebutcher
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Default Separate current transformer?

A repair café is being set up in my home town, and the organisers have sent me a guide to safety written by the original promoters. It states in slightly odd English that for safety, electrical appliances must not be plugged directly into the mains, but through a "separate current transformer" that has no earth connection, which prevents "unilateral body connection to live parts". Any idea what they mean? Are they talking about an isolation transformer?
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Old 16th Oct 2018, 12:36 pm   #2
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Default Re: Separate current transformer?

sounds like an automatically translated text.
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Old 16th Oct 2018, 12:37 pm   #3
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Default Re: Separate current transformer?

I'd guess so. A current transformer wouldn't fit the rest of the wording - but an isolation transformer would.
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Old 16th Oct 2018, 1:42 pm   #4
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Default Re: Separate current transformer?

In my opinion - yes!
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Old 16th Oct 2018, 2:21 pm   #5
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Default Re: Separate current transformer?

It sounds as though they mean an isolation transformer, with the output "floating" so that you can't receive a shock from touching one side of the "mains".
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Old 16th Oct 2018, 4:14 pm   #6
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Default Re: Separate current transformer?

I hope they don't think that using an isolation transformer makes everything magically safe. It improves safety for live chassis designs; it does nothing at all for everything else, but may give the false impression of safety to those who do not understand.
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Old 16th Oct 2018, 4:32 pm   #7
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Default Re: Separate current transformer?

Quote:
Originally Posted by McMurdo View Post
sounds like an automatically translated text.
Agreed- "separate current" as idiom-free "isolated supply"- in other words, someone's cut-and-pasted standard recommendations that were written in another language as a minimum-effort tick-box exercise! Curious- I think of Aussies as practical and down-to-earth, surely there are pertinent national guidelines that don't require the post-Babelfish technical re-interpretation.
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Old 16th Oct 2018, 4:56 pm   #8
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Default Re: Separate current transformer?

Hi Sue, I'm sure we would be interested in hearing more about this repair café when it is set up.

The current transformer could be an oblique reference to using a "clamp meter" to give an isolated feed to a DVM when measuring mains currents.

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Old 16th Oct 2018, 9:18 pm   #9
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Default Re: Separate current transformer?

I expect that the phrase 'current transformer' means a transformer of a recent design - i.e. one that is of 'current' manufacture, as opposed to something old and of possibly dubious quality. In the context of its proposed use, safety is obviously paramount - so a new / recently manufactured transformer has been stipulated.
Just an unfortunate mis-application of the word 'current' in the context of something electrical.
The transformer will almost certainly be an isolating transformer.

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Old 16th Oct 2018, 9:24 pm   #10
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Arrow Re: Separate current transformer?

Quote:
Originally Posted by G8HQP Dave View Post
An isolation transformer improves safety for live chassis designs; it does nothing at all for everything else . . .
I'll have to disagree with that statement, but I will not enter into a discussion here on that since it will take things off-topic. But you and I can discuss this in private via P.M.s - if you so wish.

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Old 16th Oct 2018, 9:29 pm   #11
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Default Re: Separate current transformer?

I wonder if it has been translated from Chinese? You can get some very odd phrasing!

Most likely an isolating transformer. However it might be an idea to find out exactly who is going to install this equipment and as stated in post #6 an isolating transformer does NOT make everything safe.

230V coming out of an isolation transformer is still 230V and still hurts (or worse)!
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Old 17th Oct 2018, 12:34 am   #12
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Default Re: Separate current transformer?

It (the conditions re the 'current transformer') will have been translated from Dutch as the "Repair Cafe" foundation is headquartered in Amsterdam (and that is assuming this set up is based on the foundation of that name - there are others).

Whoever is setting this up would be advised to get a ruling from the state electricity authority about some of the conditions regarding the electrical set-up in the"cafe".

They might not be entirely in-line with Australian regulations and practice.
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Old 17th Oct 2018, 12:57 am   #13
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Default Re: Separate current transformer?

Confusing automatic translation from Dutch, eh? I'll convey Terry's advice to the local organisers. I can understand the need for an isolation transformer when dealing with live chassis, but live chassis appliances are hardly ever seen in Australia, and they're possibly illegal now. I've never used isolation, I just test as much as I can with the power off, run the thing up through a limiter, and take care when it's on. If there's HT, I add plastic gloves and plimsolls.

Is there a previous thread about the pros and cons of an isolation transformer? It'd have to be quite large to run something like fan heater or a toaster through, wouldn't it? And if there's no earth, there's no residual current protection. The café guide to safety admits this. I'd find that worrying.

The Repair Café safety guide in question is attached below:
Attached Files
File Type: pdf RC-Working_safely_English_general_v_10_2015.pdf (1.15 MB, 27 views)
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Old 17th Oct 2018, 2:39 am   #14
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Default Re: Separate current transformer?

There is a major paradox with hot chassis sets and isolation transformers, now the primary safety from electrocution comes from the dwelling's RCD. The isolation transformer defeats the RCD.

Also, the utility value of isolating the hot chassis set with a mains isolating transformer is so that the set's chassis can be deliberately earthed. This is so earthed test instruments can be attached. Any high voltage points in the set then sit above ground and a shock to one hand through the body to earth can be lethal if the current is over 30mA, and now, with the isolating transformer present, there is no operational RCD protecting that event.

So as noted, adding an isolating transformer is not the ultimate solution to safety and in reality, if no earthed test instruments are to be attached to the hot chassis set (only isolated meters etc) it may well be safer to rely on the dwelling's RCD for safety and not use the isolating transformer, provided the RCD is tested and in known working order.

Even with an isolating transformer or an RCD, electrocution risk remains from connection to two parts of a circuit with a high enough voltage that can supply enough current to stop the heart or trip it into an abnormal rhythm.

Last edited by Argus25; 17th Oct 2018 at 2:44 am.
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Old 17th Oct 2018, 7:51 am   #15
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Default Re: Separate current transformer?

I would agree that the text is referring to an isolation transformer, a further hint is given by the phrase "Never plug a device that you want to test directly into a power socket, when the device is (partly) directly connected to the mains supply."
The reference to "directly connected" implies hot chassis equipment.
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Old 17th Oct 2018, 12:08 pm   #16
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Arrow Re: Separate current transformer?

Quote:
Originally Posted by Sideband View Post
. . . as stated in post #6 an isolating transformer does NOT make everything safe.
I certainly agree with the 'everything' qualifier of that statement, but my claim - briefly put and in summary - is that such a transformer does add a degree of safety.
I am prepared to discuss this further in a P.M. with anyone.

Quote:
Originally Posted by Electronpusher0 View Post
. . . the phrase "Never plug a device that you want to test directly into a power socket, when the device is (partly) directly connected to the mains supply."
The reference to "directly connected" implies hot chassis equipment.
I don't understand how a device can be "partly directly connected." Surely it is one or the other?
Also, to me, the phrase "directly connected" implies not using an isolation transformer and also does not necessarily imply the connection of 'hot chassis' equipment.

Al.
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Old 17th Oct 2018, 1:26 pm   #17
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Default Re: Separate current transformer?

I regard an isolating transformer as having two purposes.
Most commonly to create a "regional" earth on the chassis of an AC/DC radio or TV or to create the same at half mains voltage on a half mains voltage chassis such as a computer power supply.
The other is to make a Variac isolated.
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Old 17th Oct 2018, 1:50 pm   #18
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Default Re: Separate current transformer?

Quote:
Originally Posted by suebutcher View Post
Any idea what they mean? Are they talking about an isolation transformer?

Almost certainly. It won't be a current transformer, that's for sure.
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Old 17th Oct 2018, 6:36 pm   #19
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Default Re: Separate current transformer?

Agree with the isolation aspect. I think that is what they are trying to describe where the current is separate from the local mains. They specify no earth so that in order to get a shock you would need to contact two conductors in the device under test not just one.
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Old 17th Oct 2018, 7:06 pm   #20
Lucien Nunes
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Default Re: Separate current transformer?

Although the translation 'separate current' is garbled, the description 'electrically separate' would be correct in the terminology of UK electrical installation practice. The transformer provides 'electrical separation' from the supply and from earth, subject to certain requirements being met in accordance with the definition of 'electrically separate.'
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