The DeltaV DO VAC Isolated CHARM specifications indicate Inrush current capacity to be " 10 A for 20 ms, 2.5 A for 100 ms". I have a field device that states its Inrush current to be 12.7 A for 3 ms.
The nominal current rating of the DO VAC Iso CHARM is 0.5 Amps. 12.7 is 25 times this nominal, but only lasts for 3 ms.
I'm thinking that this should be fine.
Here is what I'm thinking:
The Field Device specifications are determined without knowledge of the DO channel circuit it is eventually connected to. It has a voltage spec (120 VAC) and its own internal resistance (? not specified), which result in a max initial current less than Infinity because Ω > 0. From this theoretical initial current, the inrush is stated to fall to 12.7A in 3 ms and on to steady state from there.
The DO CHARM also is unaware of the field devices it will connect to. It's maximum initial inrush current is a function of the same voltage spec and the CHARM channel internal resistance. The capacitive load of a field device must be such that the inrush current falls below 10 Amp in 20 ms and is below 2.5 Amps after 100 ms.
When we connect these two components, the maximum initial current will be the result of both circuit resistances in series and therefore the max initial current must be below what the DO channel circuit design would allow. It cannot be more. The twist is that the field device chose to indicate the inrush current at 3s at a value of 12.7 A. Will it be below 10 Amps at 20 ms? and below 2.5 Amps after 100 ms?
Since the Inrush current specifications are expressed at different current and time values, what is the correct approach to ensure the right IO channel product is used with field devices?
Andre Dicaire
In reply to s_brwn:
I'm struggling with exactly how close the specs actually are. I plotted what I think would be inrush curves based on the data provided:
I had to guess at what the initial current is at t0, and at what point the current reaches steady state, especially for the device as it gives us one data point. For the CHARM, In order for the curve to pass through both points, the initial current is about 14 amps and steady state at about 200 ms.
If the CHARM limits the initial max current to 14 amps, and it drops below below 10 amps at 3 or 4 ms, I don't see this as being close at all. I mean, the field device can't draw more current than the CHARM circuit allows. No?