Any obvious problems with single relay in load leg or 2 x relays in load legs

Thread Starter

cho

Joined Dec 24, 2015
6
Hello

I would like to know if there are any obvious problems in connecting a relay in both legs of the load as per diagram 2.

Thanks in adv ..... Cho
1709151829570.png
 

nsaspook

Joined Aug 27, 2009
13,279
Why do you need both legs to disconnect if the load is isolated per your diagram? It costs an extra relay and wiring if you don't need it.
 

MaxHeadRoom

Joined Jul 18, 2013
28,688
Often circuits are wired so that more than one condition is considered, and each belong to a separate circuit or power sourced differently.
The typical E-stop circuit is an example.
A circuit that includes a Dry-contact is another.
 

Reloadron

Joined Jan 15, 2015
7,517
No clue as to your location but I see 220 VAC applied to a load. Your diagram #2 would be a violation of the NEC (National Electric Code) where I am in the US because you are switching Neutral. Knowing more about what you are doing exactly would help but less that I would say not a good idea.

Ron
 

Thread Starter

cho

Joined Dec 24, 2015
6
Thanks for responding, its for a test bed so need some sort of isolation, cost is not a problem but the weight of a isolation transformer is.


thanks

cho
 

nsaspook

Joined Aug 27, 2009
13,279
Thanks for responding, its for a test bed so need some sort of isolation, cost is not a problem but the weight of a isolation transformer is.


thanks

cho
Those relays won't replace the operational safety related functions of an isolation transformer. I'm still unsure of the actual requirements.
 

MisterBill2

Joined Jan 23, 2018
18,519
If the control is for a test bed for some sort of devices, and the AC 220 volts has neither side as common then both sides should be opened to avoid live wires when the device is connected.
If it were a permanently connected device connected across the standard 230 volts in much of the US, then switching only one side IS JUST PLAIN STUPID, although it cuts costs fro the OEM.
The reason I say it is stupid is because that leaves all of the circuit inside the device, such as motor windings, held connected to the AC line and subject to every voltage spike that appears on the line.When both sides of the line are disconnected then there is an additional, fairly large, voltage drop across the open contacts, and the motor winding is protected against high-voltage acring across motor insulation to the frame, which will certainly lead to motor failure.

At least in my part of the USA, with a 220 volt supply, (usually more like 240 volts), neither side is neutral. Thus both sides must be switched because neither is neutral It is quite different from in the UK.
Two years ago my church suffered compressor burn outs for that exact reason, in that the compressor package producer used a two pole contactor to switch off only two of the three phases. So a mains transient took out FOUR of the ten horsepower compressors because of arcing to the motor frame inside the sealed unit compressor.
The cost of replacing a ten HP sealed unit compressor is quite a bit, by the way.
The price difference between a two-pole contacter and a three pole contacter of the same series wasless than ten dollars at the time. These were not "motor starters", but rather relays rated for motor staring service.
 
Last edited:

Ian0

Joined Aug 7, 2020
9,817
Hello

I would like to know if there are any obvious problems in connecting a relay in both legs of the load as per diagram 2.

Thanks in adv ..... Cho
View attachment 316455
Double-pole switching is common on 230V supplies. It improves safety in countries which use reversible plugs.
You can get double-pole relays. That would be safer than two single pole relays, as it reduces the possibility of the neutral relay opening and the live relay remaining closed.
 
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