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Anonymous Poster #1

Door Earthing - Low voltage (LV) Panel

07/12/2021 8:35 AM

In one of LV Panels, The mains rating is 100A 4P+E 415VAC.

The phase cables are 1Cx25Sqmm Low smoke Fire Retardant 1.1kV Cables

The Earth cable is 1Cx16Sqmm Low smoke Fire retardant PVC insulated 1.1kV Cables

The earth cable enters at the bottom to the Earth Busbar which is 60x3mm Tinned Cu.

The earthing cable is distributed in the busbar for many electrical components.

The Panel door is 2mm Thick and it is powder coated for 80Microns.

All switchgear components are mounted in the Mounting panel inside the enclosure which is equipotentially bonded

Question - What should be the cable size for Panel Door earthing to the Earth busbar.

In general, many panel manufacture consider 2.5Sqmm cable for Door earthing as standard.

I have not seen any standard for sizing the door Earthing.

What is the logic behind such selection?

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#1

Re: Door Earthing - Low voltage (LV) Panel

07/12/2021 2:17 PM

The logic for electrical codes is always installation site-specific. The code approved for residential Maryland may not be code approved for industrial Uruguay and certainly not for Romanian natural gas operation.

Hire a licensed electrical engineer for the location.

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#2

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 4:23 AM

The code applicable to the country of installation is the first port of call.

If in doubt, consult a qualified local Electrical Engineer.

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#3

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 4:59 AM

Back panel wiring in most LV panels is static and has no need to flex. The exception is wiring to any components mounted on the doors and this includes the earthing of the doors so it is important that these circuits are wired in multi-stranded cable similar to "Tri-rated® cable" and have enough length to flex easily at the hinge. Component wiring sizes are defined by the current draw of the mounted lamps, push buttons, meters or HMIs. Earth faults could potentially draw much larger currents but only for a very short time so the heating effect within a door earth cable is minimal. 2.5mm2 is a compromise between flexibility and robustness taking into account that LV panel doors are opened relatively infrequently. The strands of a 2.5mm2 are small enough to flex without breaking and there are enough of them to accommodate a few strands breaking without seriously impairing the earth bond.

In situations where the opening of a door (or earthed guard) will be frequent, or 2.5mm2 is deemed to be inadequate, there are flat tinned copper non insulated braided cable links available. They are usually terminated at each end with a crimped or soldered lug to allow fixing with a brass through bolt. An advantage of them being non insulated is that any broken strands can be seen and the braid replaced before this becomes a problem. If braided links are required for your panels it will almost certainly be called up in the purchase documents/panel specification.

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#4

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 5:08 AM

I am completely ignorant in this area, so I am replying in an attempt to educate myself more than anything else.

First in reply to the other two replies: I think the original poster is happy that his panel meets local standards, but, wants to understand why the door earth is allowed to be such a small gauge.

To the OP when you say "4P + E", do you mean 3 phase + neutral + earth?

I would guess that if any of the live wires gets shorted to the door then the small earth wire will easily carry enough current to trip any earlier safety device.

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#5

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 6:04 AM

What you are referring to is an equipment ground.

The main equipment ground must be sufficient to trip the main supply breaker in the event of a fault to the equipment or conduit.

It is sized by the main breaker capacity.

The entire panel should be at equipotential.

The door will have some connectivity to the panel via hinges,locks,etc,but that cannot be relied upon for safety.

The size of the bonding jumper is usually designed by the manufacturer or site engineer.

Here is a link with many more details on all types of electrical bonding. :.

https://www.electricianinformationresource.com/electrical-bonding.html

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#6

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 7:45 AM

It would be pointless for the <...cable size for Panel Door earthing...> to be greater than the cross-sectional area of the <...Panel Door...>, allowing for the possibility of different materials being used for them...

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#7
In reply to #6

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 8:50 AM

"Not knowing exactly what to say,I said nothing" Mark Twain

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#8
In reply to #6

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 8:52 AM

The CSA of the door is also irrelevant.

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#9
In reply to #8

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 10:14 AM

IP40 and above are going to have polymer door seals. The only earth circuit will be through the catch and hinges. The CSA of the catch and hinges in powder coated steel with poor contact points is likely to have a much higher resistance (Ω) value than 2.5mm2 copper wire. Where the door was large enough to have three hinges and multipoint catches we always earthed the doors at both top and bottom. CSA of the door is irrelevant.

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#10
In reply to #9

Re: Door Earthing - Low voltage (LV) Panel

07/13/2021 11:13 AM

Must be an echo in here.

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#11

Re: Door Earthing - Low voltage (LV) Panel

07/19/2021 6:57 AM

As pointed out above, local guides/standards/codes will apply, and I also agree with the comment regarding 2.5mm sq. as a minimum for simple mechanical strength for bonding, increased as required for the supply.

That said, it may be useful to consider the method of attachment to the door. Traditionally we would always use M6 minimum (again for strength) and if in doubt test it. We did tests at 50kA prospective (415V), using 100A fuses to prove our connection was sufficient and did not blow itself off under fault conditions (Icc test). This was a dead short to the door so very extreme and unlikely. The connection method compared to a 16mm sq. equivalent.

Not all studs are created equal, and they are known to be a weak point amongst test engineers.....

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