I agree.
Also, its worth finding a steel cabinet for your control system.
They aren't cheap new, but used ones can be picked up for not much more
than scrap value if you find a surplus dealer.
I've witnessed the aftermath of drive melt downs and circuit failures
and its amazing how much damage a failed drive can make inside a control
panel before a fuse or breaker trips.
Steel panels contain the fire and sparks and can prevent your house from
burning down.
Resist the urge to make a cabinet out of plywood or plastic that burns
easily.
Dave
On 7/14/2018 3:15 PM, Chris Albertson wrote:
From a safety point of view. It is best to follow rules from local safety
labs be it in the US or EU. Basically if you have a metal cabinet with AC
power inside The cabinet MUST be grounded to the earth pin on the power
socket and there are rules about using a dedicated screw (not used for
other purposes) and close to the power entrance.
If you don't like the this then the other option is "double insulated"
design which mostly means a non-conductive cabinet made from perhaps ABS
plastic.
There is good reason to tie you DC ground to safety ground at one point,
even if you use a 1M resister to do this. Many times (especially with
vacuum tube circuits but in other cases too) there are leakage paths and a
ground left floating can float up to a high voltage. So many times we find
some kind of RC circuit used, but more commonly a simpl wire inside the
power supply.
The idea is that is must remain safe even AFTER is major failure such as
insulation melting off an overloaded wire because of a fire.
You will NEVER think of everything yourself, so just follow the rules.
They are the result of 100 years of world-wide experience.
Most consumer products we buy today use double insulation. But if you are
using a metal external housing, double insulation is complex
The best plan is to pretend you are actually going to SELL the machines you
are wiring up. Keep is "legal" in both the US and EU and you are OK.
There is a strong temptation to say "It is just my own toys in my own
house and I never plan to apply for a UL Lab sticker so way bother with
their rules?"
On Sat, Jul 14, 2018 at 11:05 AM John Dammeyer <jo...@autoartisans.com>
wrote:
The power supply cabinet I'm putting together for my Mill has a terminal
strip with ground and 105VDC power (1500W) with one pair of terminals for
each axis. From that strip I run one pair to each Servo Drive. From each
Servo Drive a pair to the motor. The encoder cables enter the Servo Drives
on a different side of the board and will be not be run in parallel with
the
motor cables. Stepper motor for the knee has its' own power supply and it
too runs separately (power and ground) to the Gecko. These high power
supplies are all run with 220VAC through a separate contactor controlled by
the ESTOP.
The PMDX-126, the 24VDC and the PC/Monitor are also on a 220VAC circuit
that
is live from the Mill Master Switch. The 24VDC supply is what I'd refer to
as the instrument bus and it's used for a variety of controls like the
control side of the STMBL Servo drive and 12V supply for the DRO.. It will
share the DC ground with the PMDX-126 breakout board, PC USB and ultimately
a series of CAN bus based controls for tool changer etc.
I wanted to also run the 24V into a 15V regulator to provide power for the
control side of the HP-UHU servo drives but here is where that pesky ground
sneaks in again. Connecting a 15V supply sourced from the instrument power
that then runs the HP_UHU connects the Instrument power to the Servo Power
ground. The step/dir/error from the HP_UHU is optically isolated so I
don't
have to worry there.
The Servo Power Transformer has a 12VAC winding which through a bridge
gives
me roughly 16V which is the better source for the 15V that not only is
regulated down to 5V for the HP_UHU but also supplies the gate bias voltage
for the FETs. I'll only know if it works well enough once I put a scope on
the running system.
This is an example of where I'm not sure I see any value in ever connecting
the two DC grounds together. As long as there isn't a return path for the
high voltage DC through any part of the instrument bus DC circuit it
doesn't
matter if the high voltage DC ground became completely disconnected. But
connect both of those DC grounds to Earth (metal cabinet etc) and now the
potential for high voltage, through component failure running through the
instrument circuit, to instrument ground to earth back up into high voltage
ground is more likely. And touching that high voltage now ungrounded
section and perhaps the metal frame puts the body into the circuit.
And yet if you have, say an audio amplifier or radio transmitter with both
48V and 5V that share a common ground a circuit failure usually smokes
things. But those are all usually in a sealed metal cabinet. A system
where the various devices are placed around the equipment, not all in a box
is different.
John
-----Original Message-----
From: Jon Elson [mailto:el...@pico-systems.com]
Sent: July-14-18 8:57 AM
To: Enhanced Machine Controller (EMC)
Subject: Re: [Emc-users] Grounding
On 07/14/2018 02:37 AM, Gene Heskett wrote:
Not exactly 100% correct John. High frequencies are
generally carried only by current flow at the skin of a
conductor. And the large conductor, while having some rise
in impedance, is still the better conductor because the
smaller wire has far less surface skin area, raising both
its impedance and its ohmic effects faster than the big wire.
There's also loops. Bundling the power supply and ground
return close together minimizes inductance. Having the
supply and return form a big loop increases inductance, and
can cause greater noise as well as radiation of EMI. So, in
general, you want the return wires to follow the supply
wires closely.
Jon
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