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Inquire about using cable power and signal at the same time

I am writing because I have a cable inquiry.

Can Fathom ROV Tether (ROV-ready) cables be used as power cables like High-Power Tether Cable with Connectors for OTPS cables? I’m trying to use less than 300VDC as DC. In summary, is it possible to use Fathom ROV Tether (ROV-ready) cables as power and signal cables at the same time?
I wonder how the High-Power Tether Cable with Connectors for OTPS cable is structured so that both power and signal lines can be used at the same time.

Hi @HanJIK,

The Fathom ROV Tether hasn’t been designed for high power use, and if you don’t already have experience with high power electronics I would strongly recommend against it without close supervision - it’s very possible to be permanently injured or killed by electricity in this range. The OTPS User Manual has several relevant warnings about the dangers of the different parts of the system, and you should definitely make sure you’ve at least read and understood those before proceeding with even the design stage of a potential high-voltage tether system.

The OTPS cable structure is detailed in its Product Description. Notable differences from the Fathom ROV Tether are the thicker conductors for power transmission (23 AWG) and ground connection (24 AWG) compared to the data wires (26 AWG), as well as an additional solid polyurethane rubber outer jacket to better resist abrasion and cutting. If you look in the Technical Details tab you’ll see that the cable strength and voltage ratings are also quite a bit higher for the OTPS cable.

Technical Considerations

Maximum Electrical Ratings

From a theoretical perspective any conductors are capable of carrying current, so it’s technically possible to put power through them, and a clean DC signal is unlikely to cause significant electrical noise on twisted-pair differential-signal wires. We haven’t specified a current rating for our tether wires, so as a rough indication this resource suggests 26 AWG cables with polypropylene insulation should be able to carry up to 5A at 90^\text{o}C, but when bundled as a group of 8 in a single cable that’s derated down to 3.5A. It’s possible that the internal filler and strength fibres and waterblocking compound, and the outer insulation foam, increase that thermal insulation, or could have thermal breakdown issues, so I wouldn’t suggest going that high. Similarly, while the cable is rated to 300V that’s the maximum - it’s generally a good idea to stay below that.

For reference, the OTPS high-power tether is rated to 600V, and the current-rating-estimate resource I linked to suggests it could potentially take up to 6A (including grouping derating), but it’s actually used with 400V and a maximum of 4A is outputted from the OTPS. Both of those are 33% lower than the respective rating, which seems like a reasonable buffer for keeping things safe.

Application Considerations/Potential Technical Issues

I recently provided a relatively detailed outline of power conversion and transfer requirements and losses in the OTPS system here. Of particular note is our voltage drop calculator, which for your proposed option of using a Fathom ROV Tether would have a power transmission maximum of 890W, assuming you had a 100m tether with 300V across two pairs of 26AWG wires at 3.5A. I’m recommending power through this cable should stay below those values, so it’s very possible there wouldn’t be sufficient power transfer to use the thrusters to their full capacity (especially since that 890W then gets reduced by another 10% or so as the loss from converting down to a voltage the ROV can actually use).

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Thank you for answer.
Then, is it possible to use ROV Tether (ROV-ready) cables as power connection cables under 100VDC? If the power and signal lines are used together through this cable, will there be any problems with the ROV control?

That should be technically possible, yes, noting the dangerous potential for electrocution if it’s not done correctly.

I don’t expect there would be signal noise issues unless your DC smoothing process isn’t effective, but as mentioned you will likely not have enough power to use the thrusters to their full capacity - I’d recommend reading the power usage breakdown I linked to in my previous comment, and also looking at the T200 technical details tab to see how much thrust you would be able to get with the power you can provide.

With 100vdc you would be able to run the electronics and lights, but not any thrusters. I’m curious to know what you’re hoping to achieve - is it for a BlueROV2 or something else?
We sell a battery manager that can be used to make a hybrid DC/battery supply that will switch over to battery power if the DC power supply exceeds capacity.

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Thank you for answer.

I asked this question because I wanted to use it as a power and signal cable using a tether cable. The answer was that the tether cable is not suitable for use as a power cable.
So, I asked about the configuration to use the High-Power Tether Cable as a power line and a signal line.
Is there any problem if 24v DC power is supplied from the outside using the High-Power Tether Cable? I have confirmed that it can be used up to 600VDC according to the specifications.

The voltage drop would be too high at 24 volts. You’d probably be able to run the Raspberry Pi and Pixhawk, but not much more.

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Thank you
The cable length is about 60 m. Is it possible?

I’d recommend using our voltage drop calculator which I linked you to earlier, for the different types of cable that you’re potentially interested in using.

If you want more detailed help from us then you’ll need to provide a clear specification of what your power usage requirements are for ROV operation, and the power capacity of the power supply/supplies that you’re considering. Even then we’re likely to direct you to our existing tools so you’re able to evaluate the different options in a meaningful way yourself.

If you have questions about how to interpret or use the tool I’ve linked you to, or if you’re unsure of which cable/power parameters are important then you’re welcome to ask for further information on those.

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