Can locking of a control circuit be used for isolation?

This includes types of overcurrent, Short Circuit and RCD protection
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JamieP
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Can locking of a control circuit be used for isolation?

Post by JamieP »

Can a device be provided in a control circuit to enable equipment to be safely worked on because the main circuit can't be engaged, can such be regarded as "isolated" or must we have such a device in the main circuit?

Question was asked the other day and I can't find the relevant information to back up either
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gregmcc
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Re: Can locking of a control circuit be used for isolation?

Post by gregmcc »

AS/NZS3000:2007 2.3.2.2.1 covers all the requirements for isolators.
a control switch doesn't meet (e) so the answer is no.
most likely also does not meet (c)

there are 6 "shall" requirements, all of which must be complied with, this is why a for example the light switch in your living room is not classed as an isolator, but simply a control switch. The isolator would be the the circuit breaker at the distribution board.
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Re: Can locking of a control circuit be used for isolation?

Post by AlecK »

Most control switches, and all micro-gap switches as generally used for lights, don't meet (a) either.
But as gregmcc said, main point in this case is that if it doesn't operate in the "relevant supply" it can't be used as an isolation device for that circuit.

Also relays, and most contactors, do not qualify as devices for isolation, for similar reasons.
Those that do tend to be rather expensive items.
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Re: Can locking of a control circuit be used for isolation?

Post by JamieP »

Few additional points just for better understanding, I definitely understand what a device for isolation is and most definitely understand a device such as a standard functional light switch doesn't meet such in the slightest

The root of my question was in situations like a DOL motor starter that engages a contactor and that such contactor could not physically engage with a lock provided into control circuitry but as you seem to have pointed above, neither parts of that provide the requirements of isolation

But what got me thinking about that was an apprentice was failed on a question which I believe he answered right which claimed an E stop could be used for isolation provided that it could be locked

Maybe I am using the wrong term which I try be careful about, rather than suggesting it as isolation, maybe I need to look into 2.3.6

2.3.6.2 does seem to give me more of an indication around what I'm asking in that such device must be in the main circuit

Does any of this information change the response from either of you?
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Re: Can locking of a control circuit be used for isolation?

Post by gregmcc »

bit of difference between an E-Stop (category rated or not) and isolation, An E-Stop still does not meet all 6 requirements.
E-stop style switches have easily removable contact blocks and have been known for the contact blocks to become easily dislodged, an E-Stop usually uses N/C contacts should they become dislodged then the device could possibly re-start, so non compliant with (b), again not (e) (I have seen this happen many times when a E-Stop has been pushed and nothing happened because the contact block has become dislodged)

I've designed and built lots of industrial machinery, done lot of category rated e-stop systems and each one has always had an isolator in nearby proximity to the motor. Not going to rely on a contactor that could get stuck for sake of someones safety when working on the machine.
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Re: Can locking of a control circuit be used for isolation?

Post by JamieP »

Thanks Greg, appreciate the input and all makes total sense in confirming my belief on needing it not being suitable unless being in the main circuit
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