3 phase RCD question

[Mikemm]

Hi everyone, each leakage question here.

I am needing to run some LED screens, professional type, not TVs. These are 500mm x 500mm panels and traditionally have 2/3mA leakage per panel.

So we generally have a 63A three phase distro with a 300 mA on input and multiple outputs protected by 30 mA RCBOs. This is standard video industry stuff. For bigger systems we use 125A at 500 mA.

My question is, when I have to use a 32A three phase supply, it has to have a 30 mA RCD on input, can I pull up to 20/25 mA leakage on each phase, or is the 30 mA trip cumulative, so if I have 10 mA leakage on each phase, will it trip?

So I'm asking, can I get 75 mA leakage out of a 32A three phase supply, like I would with three separate 32A single phase supplies, with an RCBO on each?


Thanks in advance.

 

[westward10]

It is 30ma per line.

 

[Mikemm]

It is 30ma per line.

Great, thanks, so I could safely have 25 mA leakage per line / phase?

 

[westward10]

A 30ma RCD isn't designed to trip at 30ma on the dot, manufacturers allow margin for error below this and it can be variable.

 

[Mikemm]

A 30ma RCD isn't designed to trip at 30ma on the dot, manufacturers allow margin for error below this and it can be variable.

Sure, I had 27 mA in my head, but know some will go lower, I guess a ramp test would be best for peace of mind?

 

[Risteard]

Sure, I had 27 mA in my head, but know some will go lower, I guess a ramp test would be best for peace of mind?

Anything above 15mA up to 30mA meets the Standard for a nominally 30mA RCD.

 

[MJPD29]

At 25ma at the very least you'll experience intermittent tripping. As above 15-30ma is acceptable, from previous testing I usually find they trip around 20-25

 

[Lucien Nunes]

The situation with functional leakage on 3-phase RCDs is somewhat complex. Each piece of single-phase equipment will leak some current from line to earth that is purely capacitive, due to the line-earth capacitors in the EMC filters. If the equipment connected to each of the three phases is identical, then these leakage currents will be symmetrical and will cancel out, and the RCD will not see any imbalance. Each phase could leak 100mA through a 30mA RCD but it would not trip due to the symmetry. The same is true of purely resistive leakage, although that tends to be much lower in electronic equipment than the capacitive leakage.

However, are also components of the leakage current that do not cancel out. For example, 3rd harmonics add instead of cancelling, and the RCD will trip when the total of the leakage from the three phases reaches its threshold, if it is fully sensitive to 3rd harmonics. Simple measurements of the total leakage current from each phase won't therefore show reliably whether the total would trip an RCD, because it isn't possible to distinguish between components of the current that add and those that cancel.

Then there is inevitably an aspect of asymmetry. Component tolerances, different loads on the LED screen power supplies due to different displayed content, even different PSU designs in the same model screen, could lead to higher leakage on one phase that is not cancelled by the others. Then again you have the sensitivity to voltage transients. Even if the functional leakage is perfectly balanced, the more capacitive filtering there is, the higher will be any leakage spike caused by an asymmetric transient, as this could add across all phases.

So, with all these parameters, it is hard to predict whether the total leakage can be allowed to exceed the RCD trip current, or whether it would lead to an unstable setup prone to nuisance tripping. But do not be surprised if you find 15mA of leakage from each phase but the RCD still holds.

 

[James]

Each phase could leak 100mA through a 30mA RCD but it would not trip due to the symmetry

I am surprised by that, most 3 phase rcd units I have seen are 4 wire so I always assumed that they take into account current in all 3 lives and the N to sum up the leakage to earth.
are you suggesting that this is a scalable phenomenon I.e. each phase could have say 5A resistive load from phase to earth but as long as they are balanced, the rcd will not trip?

 

[Lucien Nunes]

It did look odd as I typed it and had to retrace my steps to convince myself that I hadn't overlooked something. Consider three identical resistive loads connected in star to form a perfectly balanced 3-phase 3-wire load. Everything is perfectly insulated from earth, so cannot produce any earth leakage / RCD differential current. The three currents at the load star point vectorially sum to zero and the star point will take up the same voltage as the supply neutral. Therefore it doesn't matter whether the star point is connected to neutral or not; no current will flow along the neutral conductor because there's no potential difference between the load star point and the supply star point.

If protective earth and neutral are at the same potential, then the same is true of a connection between the star point and earth. The star point could be solidly connected to the system CPC without producing earth-leakage. If we consider that the CPC has zero resistance, the three single-phase loads could be separately connected to their own CPCs without creating differential current upstream of their interconnection point, because although there are now currents circulating through the CPC they still sum to zero.

With ideal components there's no limit to the total symmetrical leakage that could be accommodated, but in the real world there are so many sources of asymmetry that one probably can't get away with very much. One should perhaps assume that only a total leakage less than I-delta-N is acceptable for practical use, but might nonetheless find that exceeding this limit doesn't always cause a trip.

Also worth noting on a different subject that for real loads such as transformers and motors, the internal star point is generally not connected to neutral to prevent circulating currents due to asymmetry; it is not true in practical applications that zero potential exists between the supply and load star points.

 

[James]

It did look odd as I typed it and had to retrace my steps to convince myself that I hadn't overlooked something. Consider three identical resistive loads connected in star to form a perfectly balanced 3-phase 3-wire load. Everything is perfectly insulated from earth, so cannot produce any earth leakage / RCD differential current. The three currents at the load star point vectorially sum to zero and the star point will take up the same voltage as the supply neutral. Therefore it doesn't matter whether the star point is connected to neutral or not; no current will flow along the neutral conductor because there's no potential difference between the load star point and the supply star point.

If protective earth and neutral are at the same potential, then the same is true of a connection between the star point and earth. The star point could be solidly connected to the system CPC without producing earth-leakage. If we consider that the CPC has zero resistance, the three single-phase loads could be separately connected to their own CPCs without creating differential current upstream of their interconnection point, because although there are now currents circulating through the CPC they still sum to zero.

With ideal components there's no limit to the total symmetrical leakage that could be accommodated, but in the real world there are so many sources of asymmetry that one probably can't get away with very much. One should perhaps assume that only a total leakage less than I-delta-N is acceptable for practical use, but might nonetheless find that exceeding this limit doesn't always cause a trip.

Also worth noting on a different subject that for real loads such as transformers and motors, the internal star point is generally not connected to neutral to prevent circulating currents due to asymmetry; it is not true in practical applications that zero potential exists between the supply and load star points.

Thank you, yes, I hadn’t thought of it as a floating star point.

 

[PeakSteve]

Great, thanks, so I could safely have 25 mA leakage per line / phase?

Most 30mA RCDs - in reality - trip around 22mA. So no, it would be risky!
You may have to consider alternative means of protection (e.g. isolating transformers?)