Inverter Tripping RCD

But, again, you don't actually know 'by how much' you don't appear to have a problem. That's why, I believe in industry they don't use RCDs on VFD drives. Please correct me if I am wrong.

In industry, they don't often use 240V single phase from a wall socket to power machinery.

True, but there are 3-ph RCDs

My last job before retiring was installing industrial washing machines in Universities and Research facilities, the wash pump and drain pumps were run by VFD's and we had to change the 3 phase RCD's to B or C class because of tripping.

Martin P

There are, but I don't understand how they work or what the value is!

In a Live/Neutral/Earth single-phase set-up an RCD trips when it detects a current imbalance between Live (in), and Neutral(Return). The imbalance can only be caused by Live leaking to Earth, which often signals a fault. Straightforward and so sensitive it can detect when the earth leak is through a human and pop before serious damage is done. Unfortunately also sensitive enough to detect high-frequency electric muck dumped to earth by EMC filters.

Presumably, because there's no neutral, a 3-phase RCD works by comparing the current in each phase with the sum of the currents in the other two? In which case, maybe, a 3-phase RCD is only worth fitting to a balanced load?

Possibly circuit breakers are more appropriate to a 3-phase system. My impression is industrial electrics favour locked terminal boxes and procedural protections rather than domestic style consumer units. Perhaps a different approach is taken because properly installed industrial systems don't have numpties and children fiddling with them!

I'm way out of my depth though!

Dave

SOD,

Three-phase RCD's work in exactly the same way as a single-phase RCD.

It does, however, have four terminals one for each of the phases and the neutral.

Due to the 120-degree phase rotation if all the phases are balanced then there is no current flowing in the neutral but if any phase is pulling more than the other two phases then there is an equal current (to the imbalance) in the neutral. The three phases are wound around the sensing coil as if they were one phase, but obviously insulated from each other.

Steve

A residual current detector (RCD) is basically a current transformer. Normally the output of the secondary is proportional to the current in the single turn primary winding. However, if two wires are used then the output will be proportional to the algebraic sum of the currents in both wires.

For a single phase system both live and neutral wires are used. If there is no leakage the currents in the live and neutral should be the same magnitude but opposite sign. So the magnetic fields will cancel out, and the output of the current transformer will be zero. If there is leakage to earth then the currents will not be equal, the magnetic fields will not completely cancel, and there will be an output from the secondary.

The same principle applies to a 3-phase RCD. There are two cases, with and without a neutral wire. Consider a 3-phase system connected in star but with no neutral. Applying Kirchoff's current law to the star node the algebraic sum of the three currents is zero. Hence the algebraic sum of the three phase currents is also zero. Whether the system is balanced or not is irrelevant. In this case a 3-phase RCD simply has all three phases running through the current transformer. Intuitively the same is true for a system connected in delta, but I don't have the inclination to prove it mathematically.

If we now look at a 3-phase system in star, with a neutral line, the same applies. The algebraic sum of the the currents at the star point, including neutral, is zero. In this case four wires (three phases and neutral) are run through the current transformer.

So a 3-phase RCD may have three or four wires, and they will work irrespective of a balanced or unbalanced system.

Andrew

Many thanks for the explanation and conclusion: it was the balance/unbalance part I couldn't get my head round.

Brought back failing to understand Kirchoff in a Physics lesson. One of several similar moments where I had to admit my maths wasn't anywhere good enough for the sort of career I aspired to. Being an idle youth didn't help either.

Dave

It is fairly simple to calculate leakage currents due to filter capacitors. It would be more difficult to calculate DC leakage as this would normally result from a fault condition. I agree that a type F is ideal for a VFD, but in practice with the relatively small VFDs used in the home workshop it probably doesn't matter. The only surefire way to know how close one is to the trip limit is to actually measure the leakage currents.

I take the pragmatic view that if something is working then leave it alone.

Andrew

If it ain't broke...................................................................................

I wish someone would tell Microsoft.