ungrounded star system 4 wires

[zezo]

for 11/.4kv transformer the lv is ungrounded star what is the effect of first line to ground fault on the ....?

1- 3 phase load (motor).

2- single phase line to neutral loads on healthy phases?

3- single phase line to neutral loads on faulted phase?

 by another way what is the effect of fault on  1, 2 , 3 and motor loads according to attached pic?

[Manon_Chaboud]

Hello @zezo, thank you for your post, we'll come back to you!

[Romain_Polette · Accepted solution]

Hello @zezo , 

Because you are in ungrouded system, any first ground fault will not impact your process and operator security. On insulated network, earth leakage current cannot loop through ground to the MV/LV transformer (Iground will be very low).

In case of a 2nd ground fault on another phase, there is a risk (for peoples and processes). You need to install:

1. an Insulation Monitoring Device at transformer side. This device will alert you when there is any first insulation fault somewhere on your electrical distribution. A maintenance operation is needed to find the defect quickly as possible. A fault locator can help you to find the fault more precisely.
2. Residual Current Device to protect electrical installation, loads and peoples in case of second ground fault on another phase.

For you reference:

• Please have a glance on the enclosed document (old but gold)
• https://www.electrical-installation.org/enwiki/IT_system_-_Principle
• https://www.se.com/in/en/faqs/FA279411/
• https://community.se.com/t5/Power-Distribution-and-Digital/Vigilohm-Ins-Monitoring-Why-use-an-ungrou...
• https://community.se.com/t5/Metering-Power-Quality/Vigilohm-Your-Essential-Assets/m-p/235012
• https://community.se.com/t5/Power-Distribution-and-Digital/IMD-IM400-calculate/m-p/409010

Regards,