I am interested if anybody has any thoughts on this:

We have just experienced some problems with a couple of Siemens inverters caused by a problem I was unaware of but, I have learned in the last few days, is well known about. The inverters are of the single-phase in, three-phase out type. When we first attempted to power the inverters up, they immediately took out our ring main breaker. We thought we must have an earth fault but after a lot of lost time and not inconsiderable amount of consternation and frustration, we came to understand that inverters with EMC filters fitted are well known to cause this problem. We have one 240V socket on our site that is not RCD protected. My colleague powered one of the invertors up from this socket with his current meter in the earth connection. He measured 110 mA earth leakage. Our ring-main breaker is set to trip at 30 mA. But then, another colleague tells me, the limit to pass an electrical safety test is 20 mA.

This raises the issue I am interested in here, which is being able to CE mark the equipment. One solution is to replace the inverters with ones that don’t have an EMC filter, but then we would be breaking those rules. If we leave the filters in, then we are exceeding the allowable earth leakage limits.

I have the feeling that someone out there has come across this problem before. Any thoughts would be appreciated.

Many thanks for that Tractor. 

Can I just check if I have understood you correctly, what you are suggesting, essentially, is to use inverters without filters to solve the earth leakage problem and then use the techniques you have outlined (controlling cable length and properly screening the cables) as the method of staying within the EMC emissions rules?

The two inverters are mounted in a stainless steel control panel, but then, the S7-1200 PLC is mounted in the same control panel, and the noisy inverters will be a problem for that.

I get it that its all a question of finding the optimum balance between the two problems, but I am not sure that the CE markings rules allow enough scope to find that balance. 

Just for the sake of completing this, these are the decisions we have taken.

1.            We will replace the inverters for ones without EMC filters to bring the earth leakage figures within the specified requirements. And oh, by the way Tractor, our equipment environment is designated ‘Laboratory’.

2.            We will use the screened cable as described by Tractor for the connection from the inverters to the motors.

3.            In future, we will mount the inverters in a separate enclosure to the PLC.

This should keep us within the specified requirements for both earth leakage and EMC emissions for the CE marking.

Many thanks to you Tractor for your advice, it was very helpful.