Technical Support on Circuit Breaker Selectivity in Ecodial

[Ahmad3]

We are currently facing difficulties in achieving full selectivity between the circuit breakers in our Ecodial studies. In several cases, the software indicates “Selectivity Limit” or “No Selectivity.”

In this regard:

1. What are the recommended measures or settings that can be applied in order to achieve full selectivity between the upstream and downstream breakers?

2. Do the cable sizes and lengths play a significant role in the selectivity coordination, and to what extent should these parameters be considered when attempting to improve selectivity results?

[Kleeh · Accepted solution]

1. The selectivity tables provided by Schneider Electric gives some tipss on the expected selectivity results. It's useful to look at it in order to choose the right circuit breakers upstream or downstream according to the selectivity result that you want.

2. Yes, sometimes the selectivity results depends on the short-circuits level which are linked to cable sizes and lengths in some extent.

[DhanyaGengadharan]

Thanks for the query. Looping @Darini TO CHECK THE ISSUE

[Darini]

hello @Ahmad3 

thanks for the query, 

to ensure total selectivity, you have multiples options, 

a. if you use the automatic solution provided by the application instead of manually selecting the breaker, the chances of having total selectivity are higher

b. another great option is to use the ECT tool:

ect.se.com

and under there, you can select- search selectivity, you can select your upstream breaker and get a list of breakers which will give you total selectivity :

you can search for upstream and downstream breakers alike.

2. you second query was regarding cable parameters affecting the selectivity studies:

Yes, the cable’s length and electrical characteristics can change selectivity results. They don’t directly change how the breakers decide to trip, but they change the fault current that reaches each breaker — and that’s what the selectivity study is built around.

Why it matters:

• A longer or thinner cable adds resistance and inductance. This weakens the fault current that flows downstream.

• When the fault current becomes smaller, the downstream breaker may take longer to trip, and the upstream one might not even “see” the same high fault level — so sometimes they coordinate better.

• But in other cases, the reduced current might fall into the region where both breakers respond similarly, breaking coordination.

In technical terms:

• Cable impedance (R + jX) directly affects the Ik values at different points in the network.

• Selectivity curves (time–current) depend on those Ik values to define overlap or separation zones.

• Voltage drop along the cable during a fault changes the energy (I²t) absorbed by each device.

So even though cables aren’t protection devices, their parameters absolutely influence the fault current magnitudes that determine whether protection devices act selectively.

hope thats clarified your query.

Regards, 

Darini H.D