Time current curves- why?

[mbrooke]

General question- why do we have different time current curves? Ie Inverse, Moderately Inverse, Extremely Inverse, ect? I've always imagined that multiples of rated current and time dial settings are enough to acheive selective coordination.

 

[romex jockey]

I'm under the impression mag trip ratings are a tad dubious in nature MBooke

~RJ~

 

[mbrooke]

I'm under the impression mag trip ratings are a tad dubious in nature MBooke

~RJ~

I guess- but mag trip in a relay is ANSI 50 and it has only on curve, while ANSI 51 (inverse time) has about 6 different curves to choose from :blink:

 

[romex jockey]

well i'm not up on that so much, but could make the leap, you have a reading source MBrooke?

~RJ~

 

[mbrooke]

well i'm not up on that so much, but could make the leap, you have a reading source MBrooke?

~RJ~

I need to dig- though this may out it into better perspective:


https://www.youtube.com/watch?v=yItihs7J9zo

 

[mbrooke]

well i'm not up on that so much, but could make the leap, you have a reading source MBrooke?

~RJ~

Here is a simple graph comparing 3 curves:


https://www.google.com/search?q=inv...UICigB&biw=1440&bih=725#imgrc=UKaO_QzJ2SEKsM:


Basically at lower current levels all 3 curves respond about the same time, but as current increases inverse curves trip much slower (ie 4 seconds at 10amps) while extremely inverse curves trip much faster (0.4 seconds at 10amps)

 

[romex jockey]

It's a start MBrooke

So the entire mod to extreeme inverse relationship comes down to the disc having more/less of a cam for the drag magnet .....?

I'm sure the electronic varaiation at the end of the vid takes far more explaination

thx for keeping it simple for me



~RJ~

 

[mbrooke]

It's a start MBrooke

So the entire mod to extreeme inverse relationship comes down to the disc having more/less of a cam for the drag magnet .....?

Almost- more or less of a cam for the electromagnet which causes the disk to spin. My guess is the more cam material the more the "bite" and the faster it spins to trip.

I'm sure the electronic varaiation at the end of the vid takes far more explaination

thx for keeping it simple for me



~RJ~

Electronic ones are another thread- but in short they do it with equations- much like a calculator a person uses.

Here is a simpler explanation, a graph putting it into perspective:

 

[romex jockey]

the selectice coordination concern is ,even to a romex jockey such as I, somewhat an issue as well.

for ex, there are many times when the main pops before the sub circuits.

yeah i know, one can overlay the mag trips ,and find them similar

it just doesn't make sennse...

~RJ~

 

[mbrooke]

It's a start MBrooke

So the entire mod to extreeme inverse relationship comes down to the disc having more/less of a cam for the drag magnet .....?

I'm sure the electronic varaiation at the end of the vid takes far more explaination

thx for keeping it simple for me



~RJ~

Actually, you might be right- the drag magnet might indeed be the one at play:





Perhaps another member could shed some light on how its done.

 

[mbrooke]

the selectice coordination concern is ,even to a romex jockey such as I, somewhat an issue as well.

for ex, there are many times when the main pops before the sub circuits.

yeah i know, one can overlay the mag trips ,and find them similar

it just doesn't make sennse...

~RJ~

At high currents you loose selective coordination- which is why POCO engineers need to know the maximum short circuit current at any given point and adjust relaying accordingly. That and you want to know if a CT will saturate. Fault current plus system X/R ratio helps in determining that.

 

[romex jockey]

hmmm, well i'll wager those POCO engineer's south of us are checkin' their math twice tonight MBrooke

~RJ~

 

[mbrooke]

hmmm, well i'll wager those POCO engineer's south of us are checkin' their math twice tonight MBrooke

~RJ~

Well, you double and triple check before putting any relay into service. Everything is verified before hand and in most companies engineers make notes as to why they choose the settings that they did. Gets even more complicated with NERC dictating and mandating things.

 

[mbrooke]

hmmm, well i'll wager those POCO engineer's south of us are checkin' their math twice tonight MBrooke

~RJ~

"Evidence retention" :roll::roll::roll::roll: NERC needs to just stop.


https://www.youtube.com/watch?v=FH5vy7dwexA

 

[romex jockey]

Some fairly advanced test equipment there

~RJ~

 

[mbrooke]

Some fairly advanced test equipment there

~RJ~

Yes, current injection, time to respond testing, under over frequency testing, loop scheme testing, phasor testing, saving of all data... list goes on. Here is the version for testing actual relays:


https://www.youtube.com/watch?v=c8TDKbtKOj4

 

[romex jockey]

there's a settable 'inverse' deal @ 2:06 , so i'd assume some coordination can be achieved MBrooke

~RJ~

 

[mbrooke]

there's a settable 'inverse' deal @ 2:06 , so i'd assume some coordination can be achieved MBrooke

~RJ~

yup- on the earth fault relay. Utility GFCI if you will...

 

[kingpb]

General question- why do we have different time current curves? Ie Inverse, Moderately Inverse, Extremely Inverse, ect? I've always imagined that multiples of rated current and time dial settings are enough to acheive selective coordination.

The more inverse the characteristic the greater the reduction in operating time at higher current values. In essence it is just a means to modify the trip curve to aid in having more choices for selective coordination between devices and/or provide a closer match to the load being protected.

 

[mbrooke]

The more inverse the characteristic the greater the reduction in operating time at higher current values. In essence it is just a means to modify the trip curve to aid in having more choices for selective coordination between devices and/or provide a closer match to the load being protected.

But why not have everything extremely inverse? I guess I can't picture a load requiring an inverse time current curve. Not saying you are wrong, just genuinely lack the experience here.