size of flex required

Emgee: yes there are terminals referred to as 'safety stop', but you need a 2 pole switch, and that wouldn't stop the independent table drive anyway

Robert's point about 5A fuse for 0.75 sqmm cable is well made, I suspect a 5A fuse might go on inrush current. I'll get some 1.25 sqmm

Finally I've tried just switching it off at the mains when going full tilt. It stops very quickly, almost as if it uses the last gasp in the capacitors to brake. This made me realise that the switch on the socket it's all fed from is easily within reach, so why am I bothering? I suppose hitting a big red button is more intuitive, and I've got it now.

Thanks for all the input

Hi Duncan

24/0.2 would be 4.5Amp.

**LINK**

I would definitely use the ESTOP fascility on the inverter.

Personally I generally overate cables purely for mechanical robustness especially when they are exposed.

regarding your comment "I've managed for years with just the vfd stop button, but this relies on software and electronics," If the stop button is the normal stop well yes I woud agree, if it's a proper Red Mushroom EStop then no. ESTOP's have rules attached regarding fail safe status. Generally they will be designed to be as separate from software and other electronics as is possible to avoid the very problem you are worried about. If your unit is for example CE marked it will comply with the rules on emergency stop cct's.

If you really want to improve your safety measures you would be better installing a foot kick stop switch. This is particularly usefull when you are using the mill as a drilling machine and have one or both of your hands busy.

Regarding eStops in general, I would be more concerned over the accessibility of the stop button rather than any remote possibility of it's failure assuming it has been designed for purpose.

regards Martin

Edited By Martin Kyte on 23/07/2020 11:43:34

Edited By Martin Kyte on 23/07/2020 11:43:56

An Estop is a normally closed button switch that breaks the circuit to the pull in coil on the machines no volt release relay. The current demand of the coil is very small. Once the relay is opened, it doesn't matter where the power is coming from VFD or otherwise. The motor will stop and not restart unless done so manually. I think no volt relays are omitted from no end of home machines. They are mandatory in industry.

While the current of contactor or no-volt relese coil is relevent to the E-Stop switch contact rating, unless there is a separate (lower rated) fuse for the control circuit, the cable must still be rated at least as high as the fuse in the plug.
If the coil goes short circuit or th cable gets crushed the fuse must blow before the cable overheats.
Indusrial machines often have a control circuit fuse - SlideLoc was a popular type- newer machines have 24V control circuits.
This fuse / cable relationship is pretty fundamental but ofeten mis-understood. Years ago as part of a safety talk at amateur radio club I'd do a demonstration with a car battery, 5A "flex" and a plug with a 13A fuse The cable length was carefully chosen to give 1/2 Ohm cold resistance, Short out the cable and after several of seconds it went up in smoke releasing acrid frumes fro the PVC. Fuse never blew because maximum current of 24A is not enough to blow the fuse befoe the cable smoked (resistance also goes up and current down as cable gets hot.).
I would not do yhis today because of health and safety, even outdoor s the fumes are pretty bad, but it left an impression.

The comment made by Brian about VFD E-stop safety is correct, they cannot be trusted as the sole means to stop a machne that could cause injury if it does not stop quickly. Martins comments on E-stop "rules" are correct, but not applicable to VFDs or similar devices, at least not any I've seen. VFDs shouls not be CE marked as they are components and components are not covered by the relevant directives. Confusingly about the only components that are CE marked are safety interlocks, E-stops and safety relays. I'm not saying the the VFD E-Stop circuit should not be used, it should, but as part of the bigger picture. On most VFDs the E-stop circcuit actually brakes the mootr to stop it as fast as possible (this puts energy into the capacitors in the drive for small drives) One good approch is to have a relay coli across the output side of the E-Stop in the power feed to the VFD with a normally open contact connected to the VFD E-stop circuit. This cuts power even if the VFD has had a sofware failure (or intereference from another machine) but also uses the dynamic braking if the VFD hasn't failed
You might think why not use one contact on the E-Stop for the mains live and one for the VFD circuit but this would be mixing high and low voltage circuits which needs special care.

Only switch the live feed NEVER both the live and neutral (or just neutral) with an E-Stop. If the neutral contact fails the machine will stop but the entire wiring will including the neutrla up to the switch, will still be live, waiting to electrocute someone faultfinding a "dead" machine.

Robert G8RPI.

Always cast a suspicious eye on the electrics of older machines. In yesteryear amateurs often fitted their own motors. Installation quality is pot-luck, ranging from gormless to professional. And NVR was common on industrial machines long before home equipment, so mechanically impressive classic machines might well be below standard electrically. (New machines are more likely to meet modern standards.) Dangerous when an old machine's rubber insulation is perished, or damaged, or the motor is damp, or the operator catches his sleeve in the chuck!

Old electrical books and standards can be dodgy too. Older wiring diagrams often show both Live and Neutral being switched and/or fused. Only later was it realised it's safer to fuse and switch only the Live feed. I believe 'both sides' was official best practice well into the 1930s, and I've seen examples in much later books. (Presumably because no-one noticed an old circuit needed updating.)

On the plus side, home-metalwork machines powered by domestic electricity don't often cause more than minor injuries. I guess it's due to relatively low voltages, relatively low power motors, working precisely, and a certain awareness of risk. Woodwork, DIY and gardening all have worse reputations! And as for motorbikes...

Dave

A no volt release relay triggered by an E-Stop stops the machine after a power failure. The machine will not restart when power is restored. It is not intended as an isolation relay. A fuse protects the supply circuit not the appliance.

I think some people are becoming scared of their own shadow!