24vDc power supply and earth

[Indy]

Hello, I have a question about 24vDc powering instruments from a control panel. My question is why do we use a 2 core cable going to the device/instrument and do not provide a 3rd for for the earth connection like would be provided if the device was powered by AC single phase?

Thanks

 

[zeusfaber]

On the odd occasion you send 24V ac to something, you use a 2 core cable too. It's about 24V not being enough of a threat to life to justify a protective Earth, rather than anything special about the signals.

A.

 

[Indy]

That’s great, thanks. Is this the same for all 24vDc circuits in that they only need a 2 core cable or are there occasions when a 3 core cable would be required with one core Earth.

Thanks

 

[zeusfaber]

There's bound to be some application somewhere that demands it for some strange reason or another. I can't think of an example though.

 

[waross]

Instrument grounding is a code issue.
Grounding an instrument may or may not be required.
Often an instrument is grounded by way of being mounted on a grounded support.
The code recognizes other grounding methods in addition to a conductor as part of the supply cable.
It depends.
One size DOES NOT fit all.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Indy]

Thanks for this. We have a drawing that shows a solenoid valve panel being powered from an adjacent control panel by 24vDc however this is shown as a 2 core cable connection but I was thinking it should maybe be a 3 core with the third core being an earth connection.

Thanks

 

[waross]

The answer may be code specific or installation specific.
In some instances the solenoid valve panel may be acceptably grounded by way of being mounted on grounded structural members.
The energy available from the 24 Volt supply may determine the applicable grounding rules.
It depends.
Not enough information.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RedSnake]

In the old days with a shielded cable, we used a 3 part cable for 24 VDC but only connect the earth in the cabinet not in the sensor or instrument, if the machine it self are properly grounded to earth, you can get EMC disturbance in the cable if you connect the earth in both ends.
That way you still get short circuit protection.
This can be done with a unshielded cables too.
Usually the minus is earthed on the secondary side.

BR A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[che12345]

Dear Mr Indy
I. " ... question about 24vDc powering instruments from a control panel. .... why do we use a 2 core cable going to the device/instrument and do not provide a 3rd for for the earth connection like would be provided if the device was powered by AC single phase?..."
C1. One of the purposes of [earthing] of an (equipment) is to ensure safety of persons ....
Per IEC 60364-4-41, hazard from touch voltage in the event of a malfunction (earth fault to frame....) can be avoided by use of voltages <50V AC or < 120V DC, ripple free.
C2. Per IEC, SELV and PELV low voltage systems can offer protection in case of direct or indirect contact. Protective measures against direct contact may be omitted.
C3. Attn: Current source for supplying extra low-voltage systems shall meet other requirements under their respective equipment Standards.
Che Kuan Yau (Singapore)

 

[Indy]

Thanks for all the replies

 

[Indy]

Thanks for the replies. Please see attached data sheet for a lamp which is24vDC. The data sheet has three connections I believe then that a 3 core cable should be provided but if this lamp is 24vdc then I am unsure as to why the earth is needed?

Thanks

 

[che12345]

Dear Mr Indy
I. "... Please see attached data sheet for a lamp which is24vDC. The data sheet has three connections I believe then that a 3 core cable should be provided but if this lamp is 24vdc then I am unsure as to why the earth is needed? "
I have the following opinion:
C1. As it is operating at 24V DC , earthing [may be] omitted. It does NOT mean that it [shall not] be earthed. There are exceptional cases, such as equipments with [double insulation ] casings, where an earth shall [ NOT] be introduced.
C2. It is advisable to connect the equipment conductive frame to earth, irrespective of the operation voltage.
C3. Earthing it would prevent the frame being charged up >50V if it is accidentally in contact with higher voltages.
C4. The same metallic holder may be used for voltages > 50V. It is prudent that an earthing terminal being provided, even in this case that it may be [omitted] according to IEC Standards.
Che Kuan Yau (Singapore)

 

[IRstuff]

Note that in the case of LED lights, in particular, they can be somewhat noisy from an EMI perspective, since they are often switched at something like 400Hz, so all that current turning on/off will generate some electrical noise that grounding of the chassis should mitigate

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[MacGyverS2000]

Who switches LED power (other than replacements for old fluorescent ballast systems), and why at 400Hz? Is there some weird application I'm missing?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

There are a lot of LED lights that are switched internally.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[MacGyverS2000]

Why? And how (though perhaps that will be answered by the 'why')?

Other than PWMing for dimming (perhaps where you were going?)... has 400Hz become sort of a non-standard standard for the persistence frequency?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

Not sure of all the details, but something to do with reciprocity of intensity vs. duration and LED driving efficiency

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[MacGyverS2000]

Well, human perception of brightness isn't linear, so perhaps they're pulsing with high-current PWM (average power is same or less than non-PWM max) so the eye perceives higher-than average brightness while retaining long life of the LEDs. I've done that before, and 400Hz would put any flicker outside the perception range. I'd need to see the application to know for sure.

Dan - Owner

http://www.Hi-TecDesigns.com