RCD Trips when Servo Drive is switched on.

[plcman]

Hi

I have been working in a place that has had to reduce its power consumption from the grid until it can be upgraded. We have therefore installed a 100A generator on site, we have connected the lighting circuits and one machine to the generator supply. For the Regs we have installed a 300mA RCD on the supply for earth protection.

The machine in question has two servo drives (Inverters) fitted which instantly trip the RCD when poewered up. As I know the RFI Filters which are on the inverter supply can cause this I have removed them from the circuit and we installed a new RCD that gives 195msec delay before tripping.

Now the RCD stands when one of the inverters is switched on but trips when the second is switched in. It doesn't matter which inverter is switched first, the RCD always trips on the second.

The manufacturers of the inverters documentation state that a Type B RCD should be installed and not a AC or A type, the manufacturer of the RCD say that there is no such thing as a B type!.

I am guessing that the charging of the capacitors inside the inverters is pulling the RCD out as if the trip is reset 3 or 4 times in succesion the system stays on.

If anybody has experience in this area, any help would be gratefully recieved.


Regards

Alan Edwards

 

[ScottyUK]

Alan,

As you are talking about 'the Regs', I guess you are in the UK.

Types A, B, C, and D are all classes of MCB tripping characteristics according to BS EN 60898. They do not apply to RCDs, which is why your supplier is telling you that type B RCDs don't exist. The drive's manufacturer is referring to an MCB characteristic.

It is possible to incorporate an RCD and MCB into one housing. The resultant hybrid device is known as an RCBO. These normally incorporate a Type B MCB and a 30mA instantaneous RCD, although other variants may exist.

If the RCD is being used for generator earth fault protection because the earth loop impedance is too great to meet the required 5s disconnection time for fixed equipment, rather than personnel protection, you should be able to use a time-delayed device which may alleviate your problem. Obviously you must comply with the disconnection time required by the Regs.

The problem is usually caused by the instantaneous RCD responding to an inrush charging current on the VFD input filter. The filter contains capacitors connected line-line and line-earth.




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Get it over with.

 

[plcman]

Hi

Thanks, I am aware of the type's for MCB tripping characteristics, and I hadn't heard of any type for RCD's but Control Techniques, the manufacturers of the inverters have this stated in there docs for RCD's and when I questioned it they were addament.

The device is indeed for earth fault protection on the generator and we have already changed the device to incorporate a 195msec delay. I have also removed the filters from the inverters.

I guess it is being caused by the inrush into the DC bus capacitors on the inverter, unfortunately I can't do without the capacitors or without the RCD.


Regards

Alan Edwards

 

[jbartos]

Suggestion: Visit

http://ee.unsw.edu.au/~std_mon/Assets/Papers/Earthing_requirements_discussion.pdf

for RCD settings.
For personal protection 30mA to 50mA
For fire protection 10 times higher.
The original posting indicates 300mA setting. This setting is no longer within personal protection mA range.
Therefore, the fire protection setting is at a question. 300mA may be increased to 500mA. If 500mA is still tripped, the level may be increased; however, the fire protection intent is somewhat jeopardized.

 

[plcman]

Hi

Thanks, but I have a regs book aswell.

Regards

Alan Edwards