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daisy

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Hi guys
Can you help with this one. Thermal/ magnetic motor overload units such as GV2, GV3, for example,[ and other makes] we have used in panels for both motor and circuit protection . Just spoke to a tech guy who is way above my pay scale and he said that they are not really for circuit protection.He said that is one of the reasons the manufacturers do not readily print the max impedances for these devices. I would be interested to here your views.
Thanks.
 
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netblindpaul

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This depends on where in the circuit they are & what they are protecting, along with the exact type tc.
However Silva is pretty much on the money
 
K

Knobhead

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  • #4
I’ve had dealings with these purely as motor protection. They also had group protection for SC.
Never had any problems with them but there again the plant gave very little trouble it’s self.
 
S

Swicade

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  • #5
Many years ago when i worked for a firm building panels for effluent plants we always used these only as a overload device before the contactor (instead of putting the older style overloads on the outgoing side of contactor). We always fitted required MCB protection before the GV unit. If memory serves me right there was an auxilary contact arrangement you could clip onto the side of the GV unit. (was afew years back and with age comes forgetfulness that needs prodding every so often)
 
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sbrown2

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  • #6
Theres no forgetfulness. I have this arrangement in my panel now on a balancing machine. I suppose there is a element of discrimination two larger gv's feeding a host of small gv's connected to the contactors. I shouldn't tempt fate but they have been very reliable.
 
T

THE DUKE

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  • #7
I have also used them as circuit protection but only for motor circuits zs value are available from schnieder but be careful as they do need backup protection. Was once called to a panel that had moeller PKZMO type motor starter that were fitted on to 1200amp busbar, motor shorted to earth and destroyed starter and took out the ones either side of it (the customer said the flames shot out top of the cabinet scared the s**** out of him) had to replace them but added mcb upstream of them.
 
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daisy

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  • #8
Thanks for the replys.
Yes the circuits in question are individual motor circuits backed up by seperate g.v,s .Then a group of g.v,s are backed up by mcb. All in control panel.
 
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silva.foxx

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  • #9
As I'm pretty thick... I don't understand the logic in having motor overloads protecting motor overloads in cascade.

When RCDs are cascaded, for example,...they are used with discrimination ie. 100mA upstream of 30mA.

This surely can't be said of non-speciality overloads (if they exist). I know you'll have a greater value amp rating upstream of the final overload, but you'll be monitoring for an overload between the two levels of overload. I understand there's SC protection in these GVs albeit the same trip curve (like C type over C type).

Please educate me if I'm mistaken or confusing the issue. :dunce:
 
D

daisy

  • Thread Starter Thread Starter
  • #10
Thanks for replying. Sorry for the delay , been away. In answer to your question about cascade there is only one G.V per motor. A group of these G.V,s are then backed up by a bs 88 or mcb. All in the motor control panel.
 
K

Knobhead

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  • #11
Things never change. Metropolitan Vickers used the same system in the late 1940’s.
A bit better made then.
 
E

Engineer54

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  • #12
When RCDs are cascaded, for example,...they are used with discrimination ie. 100mA upstream of 30mA.



You'll be very lucky to get any reliable discrimination between standard 100mA and 30mA RCD's.
For applications such as these, there are adjustable time delay and mA rating RCD's. They can be a real bugger at times to correctly set-up too!! ...lol!!
 
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