RCBO tripping on generator 1

[kallileo]

I have 3ph, 400V, 9kVA, 3000rpm air cooled generator on a trailer supplying a 3ph cooling unit (3phase compressor 2,2kW, 2 single phase fans around 0,2kW each and a 0,1kW single phase agitation motor) on the same trailer.
I have installed a 4 pole RCBO, TYPE A, 10A, 30mA and bonded neutral and ground wires on the chassis.
The RCBO trips from time to time without any reason.
What could be the problem?
If the bonding between the ground and neutral wires is removed, is the RCBO still has any meaning or it will work like a typical MCB?

 

[Parchie]

IIRC, RCBOs work by summing up the currents at the point of installation. I you develop a leakage current somewhere, the leakage current will not be passing the RCBO and there will be a difference in the currents the RCBO is measuring. The RCBO will trip regardless of whether your neutral bond is connected to ground or not.

 

[waross]

Can you draw a diagram?
Some RCBOs don't work and play well with motor starting currents.
You may have an intermittent or low current ground fault.
You may have mis-wired the circuit but probably not. Most common wiring mistakes will cause the RCBO to trip very time the load is energized.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

I agree Parchie but I have the same question. Why do RCBs all seem to have ground neutral wires whereas RCBoutlets don't?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. All the circuit conductors must pass through the CT window. That is, all hots and the neutral. The neutral in a receptacle passes through the CT or measuring circuit internally. With the breakers, a neutral wire is provided so that the circuit neutral current may pass through the CT or measuring circuit.
One of my pet peeves is the frequent use of the terms neutral and ground interchangeably in regards to distribution circuits.
This is one of the times when it matters. That wire on the RCB is a neutral wire, not a ground wire. The wire on an RCB should be connected to the neutral bus, not the ground bus. The RCB will generally function properly if the wire is mis-connected to the ground bus, but a sharp AHJ may pick it up as a deficiency if it is connected to the ground bus.
You can see in Parchie's diagram that the neutral passes through the CT but the ground connection does not pass through the CT.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Many RCDs and RCBOs have a drain earth to an internal electrostatic shield, if that's what you're thinking of Keith?

 

[waross]

Hi Scotty. I haven't seen that in North America. As you know, the common residential circuit here in North America is 120 Volts derived from one hot leg and the neutral. Only the hot leg normally passes through the breaker and is switched by the breaker.
Our Ground Fault Interrupter breakers have both a hot connection and a neutral connection. They include a neutral "pig-tail" to be connected to the neutral bus to facilitate the neutral being run through the sensing circuit. As well as single pole breakers there are two pole and three pole GFI breakers available with the neutral pig-tail.
Link
Our main panels have the neutral bus bonded to the ground bus. The GFI will work properly even if it is improperly connected to the ground bus.
Our sub panels generally have the neutral bus separated electrically from the ground bus. Connecting the neutral pi-tail to the ground bus in a sub panel may work or it may have unforeseen issues.
Connecting the neutral to the ground bus will result in the neutral current returning through the bonding/grounding system rather than through the neutral conductor. This is a code violation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Bill,

This sort of arrangement:

The thin white wire connects to the internal earth bar, or if the board contains a lot of RCBOs I sometimes group the drain earths at one or two locations to avoid having a spaghetti effect.

 

[waross]

Thanks for sharing Scotty. Another case of different sides of the pond. I haven't seen the earth drain on this side of the pond. How about you Keith? Have you seen earth drains down your way?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Hi Bill, Thanks for reminding me as to why there's a white neutral wire depending from GFI breakers. It's not the same as a GFI outlet since they already contain the neutral and the hot in the same mechanism.

I just saw a 2-pole, 240V, 1ph breaker in a panel and I don't recall it having a field neutral coming to it...

Wouldn't sum unequal if there was any pole/neutral load and trip?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Any two or three pole Ground Fault Interrupter breakers that I have seen have a neutral wire. It the white is already connected in the panel it may obvious.
If it is a 240 Volt circuit as opposed to a 120:240 Volt circuit, the neutral is not needed. It may have been cut off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Keith,

The blue wire on that image is the aupply neutral which passes straight through the device, unswitched, to the output terminal. The output terminals are both on the bottom edge, and the phase connection is tapped from the pan assembly busbar as normal. I should have been clearer in my previous post - sorry for any confusion.

We can also get 2-pole devices which don't have a wired neutral, they tap off the pan assembly.

 

[itsmoked]

I did realize your white wire was not the neutral Scotty.

That's interesting that you have 2-pole devices that don't have a wired neutral..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Sorry, wasn't sure if you'd come across that design of device before.

At least one of our manufacturers offer a 'distributed neutral' which allows a standard three-phase pan assembly to have selected poles converted to a neutral so that a double-pole breaker can be fitted to serve a single phase load connected LN or a four-pole breaker to serve a TP&N load; in both cases the breaker interrupts both the phase(s) and neutral.

 

[kallileo]

@Parchie, waross

I have attached the schematic of the connection.
I understand in that order for the RCBO not to tripped the sum of all currents should be zero (L1+L2+L3+N=0).
When there is a leakage bigger than 30mA the sum will not be 0 anymore and the RCBO will trip.
But if the Neutral and Trailer chassis and ground cable aren't bonded I just don't see how the RCBO will trip since there is nowhere for the leakage current to flow.

I think that the RCBO doesn't trip during compressor start but after maybe an hour or not at all during the cooling cycle.
It just happens from time to time.
I'm thinking replacing with a 100mA version.

Btw the genset came with 3P+N MCB and ground wire and the neutal wire not bonded.

 

[itsmoked]

kallileo; If indeed your drawing is correct I believe you have the ground in the wrong place!
Any neutral current that splits, sharing the ground, will not be summed in at the RCBO. There's your "leak". The neutral-ground bonding has to occur after the RCBO.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The wiring looks good. Have you meggered the motor just after a trip? You may have a motor failing.
Does the generator run smoothly? Grasping at straws here. Could severe vibration be causing a mechanical trip?


Bill
--------------------
"Why not the best?"
Jimmy Carter