Online assistance for electrical trade people Login  |  Register  |   Forgot Password
Assistance for electrical trade people
 

 

 

 


Click here to send Ron a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: Isolated supply transportable - bus

Rhino
Oct 16 2017 22:12

AS/NZS 3001 fig 3.1 is basically how the bus has been wired with an isolated inverter and change over switch to MSB.

The negative of the battery is grounded to the chassis of the vehicle.
The MSB earth bar is also grounded to the chassis of the vehicle.
The neutral bar is connected to the earth bar in the MSB via MEN link.

This may be a common set up.

Question when the changeover switch is in the inverter position the neutral and Chassis are connected, now the negative of the battery is also connected to Chassis, so effectively the neutral of the inverter and the negative of the battery are connected. Not really a fully isolated supply. Is this going to cause any problems? if not why not? Thanks

   

DougP
Oct 17 2017 08:19

It's just the inverter or the generator that needs to be the isolated type. The wiring system withing the structure must still configured as a MEN, and the MEN link is switched in as per the diagram.

You need to take a look at 3010 as well.
   

AlecK
Oct 17 2017 08:45

The point of having an isolated output inverter is that these types don't have any internal connection between input & output. It's not that we want to use it as an isolated supply; we don't. The point is that we can create the earth reference that's needed and ensure it's done in the appropriate way; and at same time to know there can be no back-feed of LV output onto the ELV system.

There should not be a N-E link between the bars. A connection is made between N & E ONLY when the c/o switch is in the "inverter" position. This is so that the isolated output of the inverter is converted to simulate the normal MEN supply from mains.

It's a principle of all alternative supply set-ups that the load should be supplied from the same configuration regardless which source is being used.
That keeps all shock-protection and fault-protection methods (for example) lined up. And also avoids having to use all-pole switching and over-current devices - which we'd need on an isolated supply.

With generators the one to avoid is the centre-tapped type - which many small gensets are. By insisting on either isolated output or RCD-protected; we ensure that the final output configuration (downstream of changeover) is equivalent to the normal MEN.

The other thing to note from the c/o configuration is it provides full isolation between the 2 sources, which is necessary because otherwise the mains inlet N-pin would be an "accessible live part" when running on genset / inverter.

   

Rhino
Oct 17 2017 10:42

Yes, correction on what i previously mentioned- the neutral and earth link is in the changeover switch not on the MSB.

However, the negative off the batt and output neutral of the inverter are both connected to the chassis (earth). Is that going to create any back feed of LV onto the ELV system?
   

DougP
Oct 17 2017 13:17

Rhino. They're connected together, and at earth potential on mains or inverter, so I'm not sure how you're imagining any back feed?
   

pluto
Oct 17 2017 14:46

The interconnecting the LV earthing system and ELV earthing is not correct as you exposing the LV earthing system to ELV earth fault conditions in some fault circumstances.

As the inverter and wiring details are unknown, I can only make comments in a very genral way.

It is very important and essential that the 2 earthing systems (LV and ELV) are arranged so that it is NOT possible for ELV earth fault currents to flow in the LV earthing system and vicde versa. The ELV earth dault currents are likely be a higher than the LV earth fault currents.
   

Rhino
Oct 17 2017 17:27

Pluto the negative of the batt and neutral output of the inverter(viaMEN link in change over switch) are both connected to the chassis of the vehicle(ground). Your saying this is not correct because of fault currents? can you explain this ? all circuits AC and DC have circuit protection as required..
   

DougP
Oct 17 2017 18:05

When wire as per fig 3.1 with only one chassis bond point, there would have to be multiple faults in both LV and ELV to cause fault currents to flow in the other system's earth.

For LV faults, the RCD would trip.

For ELV faults, there would have to be a fault from the +DC side, to an earthed LV item to cause fault currents in the LV earth circuit.

There could be any number of other permutations, depending on your imagination!
   

AlecK
Oct 18 2017 09:34

"They're connected together, and at earth potential on mains or inverter, so I'm not sure how you're imagining any back feed?"

Being pedantic, they're only at (approximately) earth potential when connected to external supply from mains / MEN. When on inverter supply (with external supply detached) they are at "inverter-neutral" potential, there's no connection to mass of earth and the LV "earthing" system floats.
The ELV d.c. system may or may not be tied to chassis ("earth"); usually it is, because 2.3.2.1.2 and sub-Section 7.5 allow single-pole isolation, overcurrent and switching devices. But there's no requirement for it to be.

As Pluto indicates, we need to consider not only the normal operating current paths, but also the fault current paths. While fault currents are flowing, the various parts of the earthing system will be at different potentials; but only for the short duration until earth fault protection operates.

The thing we DON'T want is LV voltages on the ELV system. Just having both systems tied to chassis won't cause any unsafe voltages to appear, or any currents to flow.
A single "earth" fault on either system won't cause this either, because provided the inverter has "isolated" output there's no circuit from +ve via "earth" to "phase" even for an internal fault in the inverter.