Three Phase + N Voltage Monitoring Relay combined with NO or NC power relay?

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Feb 5, 2024
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I would like your opinion on an issue that concerns me. I recently purchased a Carlo Gavazzi three-phase voltage monitor (overvoltage, undervoltage, phase sequence and Neutral loss), and I am a bit on a dilemma about which relay option is the right one to use. Should I use a relay with Normally Open (NO) contacts, operating continuously 24 hours a day, or should I use a relay with Normally Closed (NC) contacts, activating only when the voltage monitor detects a fault and keeping the installation disconnected? This will be installed to my house's main consumer unit.

I was considering the second option for prolonging the relay's lifespan and to have an almost zero energy consumption during normal operation since the relay's coil won't be continuously active. Additionally, this setup would potentially minimize the load on the output contacts of the voltage monitor. However, I am concerned about the reliability of this configuration. While most of the fault scenarios such as wrong phase sequence, over/undervoltage should be covered by both configurations, I'm not sure what would happen in the event of a loss of Neutral...Most likely, the relay's coil will only receive Line and with no Neutral it won't be able to energize the relay to disconnect the installation from the fault!

Your insights would be greatly appreciated.
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I think you have answered your own question. Only a permanently engaged relay can be guaranteed to release under a range of unknown fault conditions.
Requiring a relay to operate under unspecified supply fault conditions would require it to be on an auxiliary power source, such as a battery.
Just out of interest why do you need such protection on a house supply ? I assume you are not in UK as three phase domestic supplies are rare.
Thanks for the reply Geoff1946. I'm actually based in Athens Greece. The reason for the installation of such protection device is mainly to have it as an extra layer of protection combined with a surge protection device that I'll be installing as part of an electrical installation upgrade. It is not uncommon for 3-phase installations losing the Neutral (either due to a connection becoming loose or due to an external fault). I've seen it happening twice in the past 3 or so years and typically it ends up destroying mainly electronic devices and A/C main circuit boards due to the loss of Neutral, so I thought it as a good practice to have one installed just in case. I too considered the option for an auxiliary power source but quickly dismissed it due to being complicated, requiring maintenance, an inverter, etc. So, it looks like the most reasonable option is to use a suitable NO relay and hope for the best.
I recently purchased a Carlo Gavazzi three-phase voltage monitor (overvoltage, undervoltage, phase sequence and Neutral loss)
you will need more parts ,like a 3 phase contactor the same size as the main switch of the consumer unit, otherwise you will not be able to cut the supply to the consumer unit you will also need a reset switch
@binky, regarding low voltage instances here in Greece it depends. In mainland Greece and in all of the major city centers it is rare. However, on the islands it is more frequent particularly during high load periods during the summer. This is particular worse for houses located far from the transformer. The loss of neutral situation is not something that happens often, but considering the expensive damage it can cause to equipment it is something that makes sense to have as an additional protection. Occasionally, some well informed customers actually ask us about voltage monitoring relays. ;)
@poni. Yes the relay should be rated at least the same rating as the main switch of the consumer unit (40A in my case). The reset switch is not needed as the voltage monitor has automatic reset once the fault condition is not present. Though, there are relays with auxiliary bypass switches.
A NC contactor would not work if you are wanting to disconnect in the event of a phase failure, you might loose the phase/a phase thats powering the coil and it wouldn't be able to energise in a fault situation.

Also, Rather than a contactor pulled in all the time, you could consider an MCCB with a undervolt release, cut the power to the UVR and the MCCB will trip, only problem is that it will require manual intervention to reset, however if you are in an area with a bit of a rubbish supply, the fact that its off until you have assertained what is going on might be a plus point. No good if you spend a lot of time away from home and come back to a defrosted freezer though!

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