Ground Fault Protection Requirement for MV system

[philly]

Is there any special requirments for main ground fault protection on MV systems. I know the NEC requires ground fault protection on any system with a service of 1000A but I'm not sure if there were any additional requirements for MV systems.

The particular system I am looking at is a MV system with a 1200A main breaker however it only uses 800A CT's and therefore an 800A pickup. Is there any ground fault protection required in this case?

 

[ron]

Is there any special requirements for main ground fault protection on MV systems. I know the NEC requires ground fault protection on any system with a service of 1000A but I'm not sure if there were any additional requirements for MV systems.

The particular system I am looking at is a MV system with a 1200A main breaker however it only uses 800A CT's and therefore an 800A pickup. Is there any ground fault protection required in this case?

230.95 is the section covering GFP for services from 150V L-G to 600V L-L. No minimum requirement for GFP on MV systems.

Section VIII of Article 230 covers MV and HV services and Section IX of Article 240 covers overcurrent protection for MV and HV systems.

Although not a minimum requirement, it is often a good idea to have 50/51G protection of some type for MV systems.

 

[rcwilson]

As already stated, it is a very good idea to have ground fault protection on Medium Voltage systems., even though there is no specific code requirement.

Usually it is done with either zero sequence CT's that surround all phase conductors or with an overcurrent relay sensing the residual current in the common neutral of the three phase CT's. (Connecting the three phase CT circuits together adds the currents. Since Ia + Ib + Ic = 0 for balanced loads, any current flow is either CT errors or a ground fault). This relay is usually denoted as a 51N, N = derived neutral. A relay connected to a zero sequence CT is typically labeled 51G.

The trip setting is determined by the CT ratio and the relay pickup setting. Most relays have minimum settings of 0.1 Amps and some go as low as 0.05 or 0.01. With a 0.1 Amp pickup and an 800/5 CT the actual trip value for ground fault will be = 0.10 x 800/5 = 0.10 x 160 = 16 amps. A typical setting would be 0.5 Amps = 80 Amps pickup with a time delay.

One advantage at Medium Voltage is there are few phase-neutral loads that cause unbalanced currents. That means the ground fault setting can be set lower without nuisance trips.