Ground Fault Delay

[Mike01]

Recently in looking at some manufacturers time current curves the ground fault delay settings can vary from mfgr. to mfgr. one mfgr. has a maximum of 0.4 sec where others will got to 0.5 and some up to 1.0sec.

In the NEC it talks about a maximum time delay of one second for ground fault current equal to or great than 3000A

What about maximum time delays for ground faults under 3000A? We are starting to see more and more smaller frame MCCB's being provided with GF trip on the trip units the delays allow up to 1 second however the ground fault currents are typically less then 3000A on these smaller frames say 400A and below just as an example, however the NEC from what i could find does not indicate a maximum for faults less then 3000A? is this found somewhere in a UL or IEEE standard?? thanks in advance..

 

[ron]

Of course the requirement is that the maximum setting of the ground-fault protection shall be 1200 amperes, and the maximum time delay
shall be one second for ground-fault currents equal to or greater than 3000 amperes.
So the trip setting max is 1200A, but when establishing the delays, you can consider the max of 3000A, which is handled by the manufacturer.
When the trip is less than 1200A, the instantaneous gets drawn back as well, but the delay still stands.
The designer gets to pick to avoid false activation. This is equipment protection and not personnel protection.

 

[Mike01]

Ron, correct i agree with everything you stated above but lets say my ground fault is less than 3000A lets say 1500A it is still above the pick up setting but will not trip until the time delay expires. The code says if its over 3000A must trip within one second so i can delay the GFD setting for up to one second, this adjustment allow me to better coordinate my GFPU / GFD settings with downstream GF protection, however does this also apply to ground faults less then 3000A for example if I had a 1500A ground fault because it is over 1200A it will not trip instantaneously it will wait for the pre-set delay to expire first so 3000A or above it cannot be more then one second so is it safe to assume that under 3000A i can also delay for up to one second or for less then 3000A is my delay limited to 0.5 seconds? This can be critical in systems with multiple layers of ground fault the further i can push up the delay in the furtherest upstream device the better chance i have to allow the downstream breaker closest to the fault to clear first, when i have multiple GF circuit breakers in series with each other.

For example the new SE MTZ TU on their ANSI breaker have a maximum GFD of 0.4sec. where as the new Eaton PXR it looks like i can delay up to 1.0sec allowing for better coordination for multiple downstream GF protection devices.