Earthing of Mobile Machine

[Jereb]

I have a machine that is puzzling me regarding it's earthing system. Supposedly we are using an MEN system, however i'm confused over how many MEN points we have. What is happening is HV power is being supplied to the first machine (MEN point 1), then it is being brought over to a second machine (they are mechanically connected) where a second MEN point exists at the step down Tx (MEN point 2). Then power is being brough back to the first machine from the second to supply two hopper's which operate on top of the first machine on rails. I'm concerned that the hoppers earth is incorrect however i can't quite think of how.

 

[davidbeach]

MEN=?

 

[racobb]

David,

I was also confused so I googled the MEN system and found this link:

http://www.commerce.wa.gov.au/EnergySafety/PDF/Newsletters/electricians_newslet.pdf

Apparently in Western Australia before 2000 there was no requirement for bonding the neutrals and grounds at the service entrance panel, if there was a ground at all?

Is this correct Jereb?

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[davidbeach]

So how does the "multiple" in MEN apply? From the description in the posted link, it is just the service neutral-ground bond. No multiple at all, just exactly one per service or separately derived system per building.

 

[racobb]

I was actually hoping the my post would result in an additional post from Jereb with more info. I do not understand either and obviously more detail is necessary.

Jereb....can you explain your question in more detail?

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[busbar]

Just for clarification, MEN means ‘multiple-earthed-neutral’ with the North America equivalent of multigrounded neutral, described in §2 of IEEE C62.92.4-1991 Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution. I believe that this practice applies to overhead MV systems, where MV neutrals are connected to LV neutrals, particularly at transformers.

 

[racobb]

From the link I read it appeared to be implying low voltage, or maybe that is just what I read into it. It does make since for a multi-grounded higher voltage system, hence Multi Earthed Neutral. That explains Jereb's post some better, although have no clue what is being asked with no details.

Thanks Busbar

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[mgtrp]

In a multiple earth neutral system, the neutral wire is connected to earth in multiple places. In a conventional residential/commercial system, this would be at the distribution transformer, and then again in the main switchboard of each house. Downstream switchboards could also have a link between earth and neutral installed, provided they are not supplied from a distribution board that doesn't have a link installed (potentially leading to higher fault currents flowing in the downstream switchboard than anticipated).

There isn't enough information in the original post to give a good answer to the question, and I find the description of the system a little hard to follow. I haven't seen many (any?) HV systems with a distributed neutral in aussie/kiwi land, and it sounds like the loads are HV motors which probably wouldn't need a neutral.

 

[racobb]

May be the case, and I agree the we need more info. I completely understand the multiple ground concept. Might not need a neutral, but do need a ground.....but we are just guessing at this point.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[Jereb]

Righto,
I'll try and clear up some of the questions here. The system is being fed like such. HV from mobile substation feeding mobile machine 1 through several HV circuit breakers through to a 6.6kV/400V 3 phase Tx(1st MEN connection on LV of Tx). This MEN connection is used throughout machine 1 as the main earthing point.
Also from the HV circuit breakers on mobile machine 1 it is feeding 6.6kV to a second mobile machine where it also travels through a step down transformer, again 6.6kV/400V and has another MEN connnection on the LV side. Everything is fine at this point. What i am concerned about is that power is then being fed from the second mobile machine back to a set of sliding hoppers on the first machine (Think of a couple of big bins on railroad tracks sliding up and down a conveyor). Along with the 400V power to the hoppers is a combined earth neutral.
I'm probably worried over nothing but it is an earthing method that just seems a little odd so i'm trying to scrutanise it as harshly as possible prior to putting power on the machine.

 

[racobb]

Can you post a one line diagram of the system?

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[ausgen]

mgtrp,

I'm not quite following the downstream DB comment. Is this including an outbuilding arrangement? My understanding is that if it's single installation (with no outbuilding) there should only be a single MEN otherwise we'll have parallel return paths.

In terms of higher fault level, if we did have the return path through E and N in parallel, would the downstream fault not then be greater than returning through the (comparatively higher impedance) neutral only path that we'd see if there was no upstream MEN?