Alternative to MEN in Mining operations - is it a part 1 solution?

[Chaz]

Hi Guys,

I have a question WRT alternative supply system to MEN (TNCS), in mining operation we often use the IT earth system. To be specific the current mine site the 11KV used on site is supplied via Isolation transformer (5.5MVA), the secondary side has a resistor connected between the star point and earth referred to as a NER, this limits fault current of the 11KV system underground and surface to 5A to earth, particularly important with long cable runs to mobile mining equipment etc.

Further to the 11KV system the underground LV system for lights, hand held tools and general power etc (mobile mining equipment is 11KV), this LV is supplied from either an above ground or underground relocatable substation "relocatable mining equipment" as defined in ESR. This is supplied via 11KV bolted coupler and trailing cables. These substations are small 350KVA 11000/400 transformers with a distribution panel for outgoing LV. The LV side is also in the IT configuration (5A fult limit) as apposed to other LV site power for surface mining plant etc which is TNCS. This again is to limit fault current, touch and step voltage and GPR on long cable runs due to the fact it for underground.

After that woffle to paint the picture, my question is -

Due to the fact that the LV side is IT system not MEN system, does this automatically make it a part 1 solution?
In 3007 the use of NER's and alterative earthing is allowed, 3000 states "Alternatives to the MEN system may be permitted, provided that the
requirements of Part 1 of this Standard are satisfied, taking into accountany effects on the distribution system supplying the installation."

So I take that as meaning yes it is a part 1?

We as mine site are required to have a certified design as per ESR's so needing a certified design is not an issue. The thought behind my question is that if it is part 1 solution then all parts of the installation down stream of the relocatable substation would then become HRW due to 6A (2)(a)(i) even though relocatable mining equipment that supply's that outgoing supply is not HRW as per 6A(2)(a)(vii)(b).

Just makes me think how impracticable is would be to not only certify but then inspect the vast array of subcircuits down stream which are added too as expansion happens every two weeks when more DCB's and cables are added.

My thinking is that yes it is a Part 1 solution, it is HRW and needs inspection for the entire length of the circuit('s) and equipment added to those circuit('s) and every time it is added on too.

Makes me thing doing the whole maintenance management system (regulation 65) in place for the installation is the best way to avoid all this trip up's aslong as the management system meets the regulations.


Looking froward to some feed back or request for further explanation.

Cheers for reading.

[AlecK]

Short answer: yes.

Part 2 requires every installation to have an earthed N, ie the MEN system in which distribution N is tied to earth at multiple points.
Parts of the installation downstream of that can be IT (as per 7.40, but the basic earthing for the installation must be MEN; as per para 1 of 5.1.3:
"The protective earthing arrangements required in this Standard apply to electrical installations connected to the multiple earthed neutral (MEN) distribution system used in Australia and New Zealand."
The specific mandatory parts of that system are listed in 5.3.1; with the following clauses (5.3.2 - 5.3.70 providing further detail

And - as you've noted - 5.1.4 specifically ties the use of other earthing systems for the entire installation to Part 1.

The primary function of MEN protective earthing is in 5.2: automatic disconnection of supply in the event of a fault, by the operation of either an overcurrent or earth leakage protective device.

IT operates on a completely different basis.

[Chaz]

Hi Alec,

Thanks for the reply.

Yeah, it makes sense being Part 1. I suppose in practice its not so simple making the whole installation HRW due to being non-MEN, just for the fact that in mining the use of alternative earth system is common place and there are lots of additional standard to ensure safety, automatic disconnection supply, protection of reeling and trailing cables. We have so many layers of protection that normal electrical installations are not required, it's not as if you are designing a new installation to Part 1, we are just using addition standards (3007, 4781) that allow that system and have requirements built in around the use of those systems. But I if it's written in the regs then that is what we shall follow, even if perhaps not as logical.

Thanks again Alec.