[safety] Phase selector relay

Piotr Wyderski

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Sep 13, 2021, 3:13:49 PM9/13/21

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Hi,

I need to connect a single-phase load to one of two available 400VAC
phases. Permanent on state is a safe condition, the load behaves like a
light bulb. Therefore, simple contact welding is not a safety hazard.
Perhaps the only catastrophic scenario would be a short between the
phases. Will a regular SPDT relay with sufficient dielectrict strength
do or do I need a special relay? In other words, is it realistic that
the contacts become welded in such a way that energizing/de-energizing
the coil would bend the contact block sufficiently to cause an
inter-phase short, either metallic or via plasma arc?

We are speaking of 3A semi-resistive load and the switching is rare.

Best regards, Piotr

John Larkin

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Sep 13, 2021, 3:34:01 PM9/13/21

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Fuses or CBs would be prudent.

Lasse Langwadt Christensen

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Sep 13, 2021, 3:57:17 PM9/13/21

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only need one?

https://www.tme.com/us/en-us/details/apm-20/monitoring-relays/zamel/

Piotr Wyderski

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Sep 13, 2021, 4:37:34 PM9/13/21

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Lasse Langwadt Christensen wrote:

> only need one?
>
> https://www.tme.com/us/en-us/details/apm-20/monitoring-relays/zamel/

Two. Thanks, but this DIN behemoth is way too big and the phase
selection algorithm would be more complex. I need a steerable coil.

I am not sure if in an application like this I should consider a
force-guided relay with the NO and NC contacts placed in separate
compartments or if a regular relay would do.

Best regards, Piotr

Piotr Wyderski

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Sep 13, 2021, 4:38:18 PM9/13/21

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John Larkin wrote:

> Fuses or CBs would be prudent.

Sure, but it would be best not to trip them in the first place.

Best regards, Piotr

Jan Panteltje

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Sep 14, 2021, 5:46:45 AM9/14/21

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On a sunny day (Mon, 13 Sep 2021 22:37:18 +0200) it happened Piotr Wyderski
<bom...@protonmail.com> wrote in <shocq8$vbj7$1...@portraits.wsisiz.edu.pl>:

Triacs?

Piotr Wyderski

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Sep 14, 2021, 7:28:39 AM9/14/21

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Jan Panteltje wrote:

> Triacs?

2V drop and no physical gap. A relay has some advantages.

Best regards, Piotr

piglet

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Sep 15, 2021, 5:05:22 AM9/15/21

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Do not wire the phases to opposite fixed contacts of a change-over. This
is actually prohibited in some aerospace high rel specs. The problem is
an arc developed as the moving contact breaks from one fixed contact can
be stretched across to engulf the other fixed contact.

Better to use an NC/NO contact set or two separated contact sets.

piglet

Piotr Wyderski

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Sep 16, 2021, 8:30:47 AM9/16/21

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piglet wrote:

These are very good points. Good safety relays have physically separate
compartments for every connector pair and I am going to use this feature.

BTW, what are the silver screens on NC? UV ionization protection?

https://i.postimg.cc/PrZCJCFS/relay2.png

Best regards, Piotr

piglet

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Sep 16, 2021, 5:06:27 PM9/16/21

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Very curious. I haven't seen before. Seems strange as NO contacts should
be as likely to arc as NC contacts, perhaps it is to do with speed of
breaking - magnet coil action versus spring? Do you happen to know if
that relay has break before make or make before break timing?

piglet