Earthing, Metals and different voltages during faults

forum erroneous post removed

Hello,

I have a few questions on the image I have attached:
I am proposing to put a small electrical metal box within a larger electrical metal box but having them both physically isolated via a non-conductive plastic. The larger electric box would have electronic equipment that is powered from a 240v source protected by an RCD and stepped down to ELV DC. All the DC driven equipment within the larger metal box will be connected to an earth bar placed within the larger box which will also be connected to an earth mat part of a building. The smaller box would also have equipment from the same 240v source and protected by an RCD but stepped down to 18-20v AC via a Transformer unit. The transformer unit would also be metal and the earth cable from the 240v would be connected through the metal TFU and connected to the smaller metal box which the TFU is placed within. Note both the metal boxes shall be isolated via a non-conductive material.

what would be the major issues with this arrangement?
During a fault scenario if a person was touching both the small metal box and the larger metal box at the same time would this cause a serious hazard? considering once is AC and the other is DC.

Any feedback would be great

Normal earth fault conditions would apply, with the Rcd protecting the AC supplied Equipment.

I'm not up on Australian regulations, but I'm not understanding what you are trying to achieve in separating the AC and DC supplies in different containment, and why you suggest the boxes are insulated from each other, when they seem to both be earthed. Is this intended for EMC or electrical safety or something else? Or are we talking really high current?

Having two independent supplies into the cabinet, potentially from different isolators, could create a hazard for maintenance/ servicing etc. Is there a reason for not combining them into one feed?

Apologies for answering your question with other questions, but trying to understand the concept.

Avo Mk8 said:

I'm not up on Australian regulations, but I'm not understanding what you are trying to achieve in separating the AC and DC supplies in different containment, and why you suggest the boxes are insulated from each other, when they seem to both be earthed. Is this intended for EMC or electrical safety or something else? Or are we talking really high current?

Having two independent supplies into the cabinet, potentially from different isolators, could create a hazard for maintenance/ servicing etc. Is there a reason for not combining them into one feed?

Apologies for answering your question with other questions, but trying to understand the concept.

Click to expand...

fair enough mate, the reason to separate the boxes is that the smaller box will hold all the brain PCB/CPU boards for an overall system. The CPCB boards just requires AC instead of DC that's how it comes from the manufacturer. It also comes within a smaller metal box from the manufacturer for additional security. The reason they are insulated from each other, so we don't have a situation of circulating current if there is ever a fault because the smaller metal box has a power supply unit which is also metal and is directly connected to the switchboard earth bar. The larger box has an earth bar which is also connected back to the MEB which is then connected back to the switchboard earth bar.

Yes both supplies are from the same switch board but from different CBs and RCDs, both are taking 240v internally, but secondary side is different based on the PSU used for each end equipment.

My main concern is the touch potential during faults as a person could end up touching both metal box at the same time and close the circuit.

Thanks for the explanation. It seems to me that the touch potential issue you ask about is created by the decision to have two independent mains supplies. As an extreme example (v. unlikely!) a dangerous fault would be if the CPC/ earth wire feeding the small box became live somehow.
It's not exactly the same situation, but the concept of having "two mains leads" is forbidden in the field of medical electrical equipment, because of the possible hazards it introduces. That's probably the reason I've come to believe it's a bad idea.

A more normal approach is to have a single mains feed with power distribution and protection devices within the equipment.
Are you proposing the arrangement you've drawn because it's 'convenient', or are there compelling reasons not to have a single supply into the large box, and take from that a protected feed to the small box? You could then have a grounding arrangement for the equipment that couldn't be so easily sabotaged by an issue with one of the supplies?