I am replacing some RUT955 units with RUT956 units in industrial automation systems powered by 24 VDC UPS with an earth-fault monitoring system. The RUT devices are mounted using the DIN steel mount PR5MEC00.
There appears to be a change in the power-supply design: on the RUT956, GND is tied to the chassis, whereas on the RUT955 it is isolated. As a result, when the RUT956 is connected to the 24 VDC supply with earth-fault monitoring, the system triggers a fault situation.
There are some concerns regarding the mixing of signal ground and chassis ground; however, it is possible that this design choice was made intentionally and considered beneficial by your designers. Nevertheless, I wanted to make you aware of this behavior.
As a workaround for others, I supplied the new units through a 24/24 VDC isolated power supply in order to maintain the integrity of the earth-fault monitoring system for the rest of the facility.
Just so I fully understand the situation could you clarify the following:
What do you mean exactly when you say that the ground is tied to the chassis? Are you referring to the 4-pin power connector on the device, for which, the ground pin is in the top right corner? If so - it has been this way and has never been changed.
What kind of fault situation is this triggering, could you be more specific if possible?
If images are relevant here - please do provide them as if this is something critical, I’d like to pass this feedback to the team.
Hi Matas and thank you for the detailed follow up!
1 and 3: As you asked for more details i did open a RUT956 that confirm a design change compared to the RUT955. In the annotated image the RUT955 and 956 is side-by-side. There seams to be pads on the PCB which grasp into the chassis sliders which are connected to the GND for the RUT956 but on the RUT955 the sides on the PCB are isolated.
2. For UPS systems that are ground fault monitored, eg. both + and - are insulated(IT-network), this design of the RUT956 may cause issues making i connection between -/GND and ground trough the chassis. I am most familiar with Bender Isometers which in this case gives alarm without adding isolation power supply before the the RUT956 (workaround) https://www.bender.de/en/products/insulation-monitoring/
Maybe there is good reasons for grounding the chassis like this but for industrial applications with high integrity related to power/signals i would think isolated GND/chassis is preferred.
For my own learning it would be interesting to know you design considerations
Thank you for getting back to me so quickly. Oddly enough, this is not mentioned (or at least I could not find it) in the Product Change Notifications page, unless its because RUT955 & 956 are 2 separate models, that would make sense as to why.
I will inquire with our R&D as to why this change has been made exactly, and if you’re interested, I’ll get back to you as soon as I have an answer.
Thank you for this short answer to a complex design issue relating ESD, ground loops, EMC and several other interesting topics with respect to power and ground.
Other than that keep up making these amazing and versatile devices
