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Posted By Topic: dual supply isolation

Oct 29 2019 09:56

we have come across a situation where after isolating the 3 phase supply to and electrical cell, the supply voltage (240v) is fed from a different location meaning there is still voltage present after isolation. In some instances the control voltage is dead until the program calls for certain banks of fans to run and they then become live which is even worse. I have seen a paragraph before about dual supplies but cant seem to find it. Does anyone know of the regulation im looking for that states one point of isolation must be used for all votages?

many thanks

Oct 29 2019 10:10

The rule you're looking for doesn't exist, unless you're referring to a "common neutral" situation (where more than one circuit shares the same N). In which case look at

The other requirement that may apply is para 4 of; which requires a warning where there are live parts connected to more than one supply (as there would be for a source-election switch).

But if what you've got is different items / groups of equipment that each have only one supply, but no item of equipment has any of its live parts connected to more than one supply, then it's no different from a switchboard with two submains (eg lighting & power), or a bank of light switches with several feeds.
Very common for industrial; where the control circuit for a motor (or whatever) is often separate (ie a different supply) from the load circuit

Test before touch.


Oct 29 2019 10:50

I know we've discussed this before Alec, but I'm just going to comment again that my opinion on the wording of, and my recollection of dealing with the situation previously, differs from yours.

Your statement is that the "live parts connected to more than one supply" you claim that "parts" refers to an individual part with more than one supply connected, such as a changeover switch.

My opinion is that the "live parts" referred to in the clause can be separate parts. Each use of "live parts" in the clause, and in the text "the need to isolate those parts" is simply read as "parts" being the plural of the noun "part".

The word "part" is obviously not being used in verb form, as in dividing in two, where the third-person singular simple present form would be "parts", so the only possibility is the plural noun.

Basically, my opinion is that any equipment or enclosure has parts connected to more than one supply, requires the notice to be fitted. Maybe the labelling requirements for switchboards with solar supplies would back this up? (but I haven't checked in detail).

And even if my opinion is wrong, I think it's safer than not having the notice.

Oct 29 2019 12:44

A typical example in a domestic installation with a grid-connect inverter which provides one source of supply under some comditions and the mains supply as s second source of supply.
AS/NZS 4777.1 requires that the main switchboard be labelled that there are two sources of supply.

It can also occur when a generatig set is installed, one supply is the gentrating set output and the other the mains supply.

While as/hnzs 3010 does not go into great detail it is point that could be expended on in AS/NZS 3010.

AS/NZS 3000:2007 (the current cited copy)does not go into great detail it is point that could be expended on in AS/NZS 3000.

Oct 29 2019 16:03

I accept that DougP's interpretation can be taken from the words. It shouldn't be possible for two such different interpretations to be drawn from the same words. But the words are what they are, so unless / until this point is clarified, each of us has to decide for ourselves.

I doing so, we are entitled to rely on some fundamentals of interpreting Standards (and other formally-written documents). One of these is that the same wording always means the same thing; while different wording means something different.

Another is that the words are never just flung together, they are always chosen with care (albeit, with hindsight, sometimes not quite enough care). They are generally chosen to be the simplest way of getting the intended message across.

This particular form of wording, about what "live parts" are connected to, occurs only once in over 600 pages of "3000".

If the intent had been simply that a notice was required whenever there were multiple supplies present within an enclosure; there are much simpler ways of saying so - without any need to mention "live parts". Therefore the fact that these particular words were chosen gives a strong indication that something more complex was intended. The only reason I can think of for choosing to refer to "live parts" is that the intent related to live parts that are connected to multiple supplies, and not to simply the presence of multiple supplies.

Regardless, the main point (WRT the OP) is that there is no general requirement that everything within an enclosure must be able to be isolated by a single action.