GE Multilin Relay Pickup Question

[adamscb]

Forum,

We have a lot of GE Multilin relays in the plant I work in, and the pickup/instantaneous trip set points are defined as "___ x CT", for example the phase overcurrent pickup setting on one of them is 0.40 x CT, and the instantaneous setting is 1.40 x CT, my question is when it says CT, does it mean the CT primary rating, or the CT ratio (primary rating divided by 5)? Thanks

 

[Jraef]

Forum,

We have a lot of GE Multilin relays in the plant I work in, and the pickup/instantaneous trip set points are defined as "___ x CT", for example the phase overcurrent pickup setting on one of them is 0.40 x CT, and the instantaneous setting is 1.40 x CT, my question is when it says CT, does it mean the CT primary rating, or the CT ratio (primary rating divided by 5)? Thanks

Primary rating.

 

[adamscb]

Primary rating.

So then how could it tell if the current draw was 1.40 x CT? Wouldn't the CT be saturated at that point? My understanding is that current levels higher than the primary rating, the secondary output of the CT would always be 5A?

 

[GoldDigger]

So then how could it tell if the current draw was 1.40 x CT? Wouldn't the CT be saturated at that point? My understanding is that current levels higher than the primary rating, the secondary output of the CT would always be 5A?

A CT will saturate, but I don't think it starts saturating that close to the full scale input rating.

 

[adamscb]

A CT will saturate, but I don't think it starts saturating that close to the full scale input rating.

So for example, when a 350:5 CT sees 490A, the secondary output will be 7A? Is there a general rule of thumb when a CT starts to saturate?

 

[mivey]

So for example, when a 350:5 CT sees 490A, the secondary output will be 7A? Is there a general rule of thumb when a CT starts to saturate?

4x but you should look at the curves and burden as you can go much higher. I was expecting jraef to reply as I am not feeling long-winded (not that he is).

 

[Ingenieur]

Those relays are goofy
I have a test set that I train inspectors with
Assume 400:5
the ct ratio setting is based on 400 or prim

the trip setting on the absolute 400/5 or 80
so if you want to trip at 200 the setting is 200/80 = 2.5

with this type of relay (electronic low burden) you won't have saturation issues until 10x at least

 

[rian0201]

Forum,

We have a lot of GE Multilin relays in the plant I work in, and the pickup/instantaneous trip set points are defined as "___ x CT", for example the phase overcurrent pickup setting on one of them is 0.40 x CT, and the instantaneous setting is 1.40 x CT, my question is when it says CT, does it mean the CT primary rating, or the CT ratio (primary rating divided by 5)? Thanks

much better refer to the relay manual or test the relay.

i say this because some relays of the same brand may use primary rating, or secondary.

for ct saturation, one can estimate using ieee standard or iec. but much better if tested.




Sent from my iPad using Tapatalk

 

[Ingenieur]

I have tested/demonstrated dozens of times with a multi-amp
ct ratio = primary, eg, 400 for a 400:5
trip setting is based on the abs 400/5 = 80
if a trip of 600 is desired the setting is 600/80 = 7.5
kinda makes sense as a mult of ct full load current

it's the only one I've seen that uses different for setting the ct ratio and trip
did not believe until I tested it

 

[Bugman1400]

When the CT saturates is directly related to the CT Class. Its typical for CTs connected to 269 or 369 Multilins to saturate because they are a class C100 or less. I think the overcurrent settings are set to pickup at a level before the saturation occurs. So when the sat takes effect, you have enough to trip the relay without having an excess of secondary current on the CT wire that could make it dangerous.