Earth Fault Protection on Low Voltage System
Earth Fault Protection on Low Voltage System
(OP)
Got a query,
We have a medium voltage system (13.8kV) stepped down to 0.415kV via an oil immersed transformer. The LV system has a Main ACB of 3200Amp and Sub-breaker of 4 x 800Amp distributed to 4 tenants. It is currently installed with an Earth leakage relay ("ELR") for each sub-breaker. Problem is we are getting nuisance tripping via the ELR so we decided to have it checked. The recommendation is to replace the protection with a Earth Fault Relay ("EFR") instead of the ELR for reason that the ELR is too sensitive (commissioning tests show that the sub-breaker trips with less than 1A current injected) and that ELR is usually used for final circuits to protect personnel.
The consultant recommended replacement so that the protection setting may be made higher (15%-20% of the sub-breaker rating). Can anyone suggest what brand of EFR to use including the ZCT's compatible to the relay? I can provide the single line diagram if anyone is interested to help me.
Thanks
Arvin
We have a medium voltage system (13.8kV) stepped down to 0.415kV via an oil immersed transformer. The LV system has a Main ACB of 3200Amp and Sub-breaker of 4 x 800Amp distributed to 4 tenants. It is currently installed with an Earth leakage relay ("ELR") for each sub-breaker. Problem is we are getting nuisance tripping via the ELR so we decided to have it checked. The recommendation is to replace the protection with a Earth Fault Relay ("EFR") instead of the ELR for reason that the ELR is too sensitive (commissioning tests show that the sub-breaker trips with less than 1A current injected) and that ELR is usually used for final circuits to protect personnel.
The consultant recommended replacement so that the protection setting may be made higher (15%-20% of the sub-breaker rating). Can anyone suggest what brand of EFR to use including the ZCT's compatible to the relay? I can provide the single line diagram if anyone is interested to help me.
Thanks
Arvin






RE: Earth Fault Protection on Low Voltage System
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RE: Earth Fault Protection on Low Voltage System
RE: Earth Fault Protection on Low Voltage System
RE: Earth Fault Protection on Low Voltage System
RE: Earth Fault Protection on Low Voltage System
Yours truly
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RE: Earth Fault Protection on Low Voltage System
At the subfeeder breakers, you may set the 50G elements if they are directly connected to transformers and the primary transformer winding is connected delta.
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RE: Earth Fault Protection on Low Voltage System
ScottyUK, have browsed the site you mentioned but it seems that they offer ELRs and not EFRs
RE: Earth Fault Protection on Low Voltage System
I am wondering what you are using as a distinguishing factor between the two tyes of relay. For example, a 20A fault to earth for 5 seconds on a solidly earthed LV system is significantly more than 'leakage' - that is well within the range of adjustment of the Vigirex 'earth leakage relay'. Perhaps you can clarify your requirements in terms of time and current? Are you looking for a relay simlar to the 50N/51N on the transformer to use in a dedicated role for the sub-distribution?
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I don't suffer from insanity. I enjoy it...
RE: Earth Fault Protection on Low Voltage System
As long as the 51N relay at the incomer circuit breaker is have time graded with the ground fault element of the downstream subfeeder circuit breakers by 0.35~0.50 second, it will be acceptable and the 51N will serve as a backup protection for the sub-feeder ground protection. Applying a proper current tripping interval (CTI) between these protection elements will provide you the selectivity and reliable protection you need.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Earth Fault Protection on Low Voltage System
A lot depends on your earthing arrangement of your transformer - or the value of your earthfault currents. There are a lot of different methods, like an earthfault relay in the residual connection, sensitive earth fault protection, standby earth fault protection, etc. Each one have some advantages, but also some disadvantages, some may be better in solid earth connections, while other may be better if used with resistance earthed methods.
Another factor is your working environment and regulations. An example is like in Industrial and Mining installations, safety codes specify (at least in South Africa) that any voltage above the range of 25 to 40V is considered to be unsafe - and must have earth leakage protection. (for touch potentials)
To answer your question - it depends. You should consult someone with some practical experience and knowledge of your country's regulations to assists you.
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RE: Earth Fault Protection on Low Voltage System
• specification/Manufacturer/model/rating of Incomer and feeder breakers.
• Specification, setting, input sensing(ZCT or residual connected CT), CT/sensor ratios
• What type of loads are fed by feeder breakers (Motor/transformer/LV distribution board)
In normal practice, Protection of low voltage feeder breakers may be provided by following means;
• Integral solid state breaker trip devices. They shall be supplied with time delayed ground fault trips. Instantaneous trips shall only be provided for dedicated feeders where there is no downstream protective device.
• Residually connected, Time delay ground O/C relays. The time element shall be coordinated with downstream and upstream ground protection relays.
• Zero sequence CT connected, Instantaneous ground O/C relays. Normally used where no co-ordination is required with downstream system. Applicable to motors and transformers with Delta connected primary winding.
Hope this help
RE: Earth Fault Protection on Low Voltage System
02101972 - ours is the latter, but enclosed in one Main Distribution panel, and the sub-breakers are MCCB type with ground fault unit. Understood regarding the protective relaying coordination, what really needs to be addressed is the current ELR of the MCCB's being too sensitive.
RalphChristie - thank you for the inputs, yes, we are currently sourcing the proper contractor to do the work. Its just that I need to verify it from my end.
Electrifier - hope someone can tell me how to provide additional details by submitting some one-line diagrams as well as pictures here at the site so that everybody can have a better understanding of the system
RE: Earth Fault Protection on Low Voltage System
This how to post pictures in your post. FAQ238-1161 itsmoked
respectfully