As you might be aware BambuLab issued a recall for the BambuLab A1 3d-printer. In particular, the issue is the mains-voltage (230V AC) heat bed cable.
As a resolution, they offer two solutions:
- Ship the entire printer back to them and receive a replacement printer.
- They mail a new cable and you install it.
What BambuLab doesn’t mention at all is the test according to EN 50678 (Verification of the effectiveness of protective measures of electrical equipment after repair). Unless you can perform this test I would recommend choosing the printer replacement.
Not a bambu owner (and unlikely to ever be one), but read up on that out of curiousity.
I guess they’re offering self replacement as cost cutting measure, but wouldn’t be surprised they’ll stop doing that once their legal team fully understands the impact.
This problem is a fire hazard which is unlikely to be solved by replacing it with a same spec cable, so even if you can replace it yourself I probably wouldn’t, just to make sure there’s no arguing over liability if this thing catches fire. I’d assume they’ll do a longer term fix with a more stable cable, possibly with added strain relief - though I also find doing a 230V heatbed a questionable design choice.
First of all: BambuLab did overall a good job on safety (ignoring this oversight). I like it.
I’d assume they’ll do a longer-term fix with a more stable cable, possibly with added strain relief
You can do it safely. It is just that this is a very unfortunate combination of a cheap PVC-cable and the entire printer weight putting stress on it during installation (laying on the back, plugging in the wires on the underside). I assume the printed clip is to limit the bending radius to prevent the issue. Not exactly a strain relief but also kind of a strain relief. Call it what ever you like.
though I also find doing a 230V heatbed a questionable design choice.
Do you like 230V@3A or prefer 24V@28A? I prefer the lower current at a higher voltage.
Keep in mind that even the best-in-class cables have a bending radius of 4x diameter. With 28A current at 80°C ambient temperature that’s a thick cable (Without looking it up I guess 4mm^2 conductors. Roughly a 12mm thick cable) with a bending radius of somewhere around 100mm (200mm diameter) or so. With 3A it can all be done nicely with cheap cables in less than 50mm bending radius.
DC adds an extra layer of isolation if something goes wrong, and an extra place for it to fail safely in a nicely enclosed metal box. It takes a really catastrophic failure for a 28A DC power supply to go much beyond 28A for very long. A mains supply can do it all day long unless there’s some other form of protection like a fuse or isolation transformer.
From my experience contact failure is the most common cause of failures: 3A can go wrong. 28A is a bad day.
Similarly, a fuse won’t protect at all against contact failures (the fuse doesn’t care as the current doesn’t rise) and active monitoring isn’t found outside the industrial applications.
To provide some numbers on cables: A German company specifies their drag chain cables for 10 mio. cycles and they maintain a test lab to verify it and get the data required for the lifetime calculator. QC is excellent as well. In other words: Very unlikely the cable will be the issue. Assuming you see it as an issue there is always the option to use double insulated cables (no short or exposed conductors; designed to fail open). A TPE insulation rubbing through is also highly unlikely.
The contact points (connectors) are challenging. These conductors need to connect to another conductor, PCB or ceramic element so that it can withstand the vibration, acceleration, corrosion, etc. The higher the current is the lower the contact resistance needs to be the more it is susceptible to small changes.
Another problem with high currents is their electromagnetic radiation.
though I also find doing a 230V heatbed a questionable design choice
How so? There are a number of other printers that also use mains voltage to heat the bed without issue. As your bed size and thickness grows, running stepped down DC makes less and less sense.
Unless the heated bed is UL listed, I wouldn’t trust a live high voltage heater in my house. And I might not touch it unless the printer is unplugged.
They already undersized the wires to save money. Not a great start to gaining trust for a high voltage device.
The Voron printers are a good example of conservative design philosophy. Trident and v2.4 both use AC mains because the bed is relatively stationary. Voron V0 uses DC because it is a small bed. Voron Switchwire uses DC because it is a bed slinger design and moves around so much.
Bingo!
With lower voltage DC you can only set the house on fire. With high voltage AC you can set the house on fire and electrocute people. In a safety oriented company you’d try to limit the parts of the device carrying 230V (or, more generally: if your device has dangerous bits, you try to keep those bits in as few places as possible, as that limits teh amount of places you need to keep safe). Now obviously this has limits - like the mentioned bed size - but I don’t think we’re yet at a point where this should overrule safe design principles.
I haven’t seen a bambu printer myself yet - but given that the cable is undersized and not protected against side effects from bed movement I’d bet they also skimped on on making everything carrying 230V safe - in which case this is a cheaper design. I’m reasonably confident that a safe 230V heating design for a printer that size would not give you noticeably cost savings over a DC design, if at all.
I’m certainly not trying to vouch for the safety of Bambu’s designs and am only coming at this from owning/building a 350mm^3 Voron 2.4 that uses mains voltage to heat its 8 pound 355mm x 355mm x 8mm bed. Granted, my 2.4 has a fixed bed and the A1 doesn’t.
