Explanations

Channels isolated or not? That is the question!

For multi-channel terminals such as the EL3xxx/ ELM3xxx or equivalent electronic measuring devices from Beckhoff (as EPP35xx), one particular property is quite essential and, depending on the application, requires intensive attention: the electrical isolation between the channels. There are only two classes:

• The channels are not electrically isolated from one another:
• They have at least a common ground potential in the device, which is sometimes also accessible from the outside via line connection or power contact.
  
• Such devices are usually simpler (and thus cheaper) in their internal construction than the following class. However, the fact that the channels are electrically coupled leads to effects that must be considered. This applies to both static (DC) input signals that "drag along" a secondary channel (in the device) via ohmic coupling (common mode effect), and dynamic (AC) signals that also "drag along" or inductively/capacitively overcouple (crosstalk).
• Industrial automated machines and test benches move in a spatially delimited, normally controlled space and usually have a common power supply, as a rule a 24 V DC network with a common ground (0 V). With proper wiring, potential differences greater than 30 V do not occur in most applications. For this reason, most analog channels for industrial use (also in the case of Beckhoff) do not have complex electrical isolation but are designed to operate as specified with common mode differences up to approx. 30 V (common mode voltage).
• Most analog channels from Beckhoff are constructed without electrical isolation between one another in the device.
• The channels are electrically isolated from one another:
• They are completely isolated from one another; their grounds and supplies are not connected to one another in the device and device plug. But insulation also has a limit, which is quantified in the so-called insulation voltage or electrical isolation.
  
• Such devices are more complex in their internal structure than the representatives of the aforementioned class. The big advantage, however, is that at least the "ohmic" common mode effect (the "dragging along") of the DC signal (see note) is negligible as long as the electrical isolation is not exceeded. But even with this concept, crosstalk from AC signals can be noticeable, although basically many orders of magnitude less pronounced than with non-isolated channels. If the channels are reconnected outside the device on the application side (common ground, common supply), the concept is bypassed, and the effects mentioned above occur again.
• The channels of such devices behave independently of one another and are therefore easier to use, fewer considerations have to be made with regard to potential differences - however at the cost of a significantly higher price. They are therefore often used in test benches and complex measurement scenarios (stacked battery cells, high voltages).
• Beckhoff offers some EL3xxx/ELM3xxx analog input devices with electrically isolated channels for special applications, such as the EL3174-0002, EL3314-0002, ELM3702-0101, ELM3102-0100, etc. Note the insulation voltage specified in each case and the quality (functional insulation, double insulation, basic insulation) in which it is designed.
• Mixed structure
• Both classes can be mixed in one device by combining the channels into groups, each with a common ground.
  

Note: Regardless of whether and how the channels are isolated from each other, it can always be assumed that the field side and the communication side are electrically isolated from each other in the case of automation technology equipment.

• For EtherCAT components,
• field side means digital or analog inputs/outputs, RS232/485, Ethernet, ...
  i.e. everything that is connected "from the field" = from outside.
• For EtherCAT Terminals,
• communication side means "inner" part, EtherCAT backplane communication "E-bus",
• For Fieldbus Box modules,
• communication side means "inner" part, EtherCAT connections IN/ OUT.
  

The magnitude and quality of this isolation are specified in the technical data.

The isolation of the field/bus side also allows the channels of devices that have no reference to the system supply on the field side (e.g. because they have no power contacts) to be operated as isolated islands. With 1-channel analog measurement modules, this creates electrically isolated channels.