push buttons control cable length limit 1

[bamboo123]

Dear Experts,

LCS is located near its low voltage motor pump. the distance between LCS and MCC is around 600 metres. any requirement/standard/code/etc which mention maximum cable length for control circuit (using AC 230 V) between local control station (LCS- push buttons -start/stop) with its MCC?, in order the LCS is working properly/correctly. (i think voltage drop is one of factor that shall be considered).

regards
roni

 

[Skogsgurra]

There are two problems.

The first is that voltage drop can be a problem if your controlled object is a contactor coil. That obviously is not a problem, yet. But 600 m means 1.2 km wire and if the cable has like 1 mm2 area, you will have around 20 ohms resistance in series with the coil. That is OK for small ones, but you will get problems with larger contactors due to the coil's starting current.

The second problem is that the contactor may not drop out when the contact opens because capacitance in the cable is around 60 nF and that may keep an already pulled in contactor closed when contact opens. The risk is especially pronounced if there is an RC snubber across the coil. The snubber usually reduces the inductive current in coil so i stays pulled in at very low currents.

I think that I would use DC and a sensitive interposing relay for that application. Especially if safe operation is important.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[dpc]

Totally agree with Gunnar, especially his second point. You might do a search on this forum. This has come up before. But bottom line is that you will have to use dc interposing relays for this circuit.

 

[bamboo123]

@Gunnar & Dpc, thanks for your feed back/suggestion.

 

[itsmoked]

Here's a couple of examples:
thread238-217821
thread237-102363

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BJC]

Good paper on this subject.

http://static.schneider-electric.us/docs/Motor Control/Lighting Contactors/M-379F.pdf

 

[ScottyUK]

DC input solid state relays can be a really effective solution to this problem. With a nominal 24V drive voltage they are more than capable of operating over a mile from the pushbutton station, even with relatively light gauge conductors. I have some notes at work when I get in tomorrow - I'll see if I can find a copy to scan in.

 

[itsmoked]

That's a good idea Scotty since 1)SSRs take nothing to activate and 2)the DC bypasses the capacitive issues.

Things to look out for on SSRs are 1)They can fail ON easily. 2)They can fail half ON causing real problems. I could imagine a local lighting stroke turning on an SSR mysteriously if not briefly.

If the system is safety oriented I probably wouldn't use SSRs.

Keith Cress
kcress -

http://www.flaminsystems.com