Cnc machine bonding to structural steelwork

[mort2376]

The factory I look after has just taken delivery of a new Cnc machine.

I was there today to wire it up, no problems 4core 10mm swa done.

The filter said that the machine needs a separate earth bond to the structural steel work. I said that's not my problem as its part of your installation as agreed. However I have a feeling I am going to end up doing it.

Before I left I did a quick check and measured from MET to the steelwork and got a reading of 60ohms, so I am guessing that the steel is not bonded ( factory is over 100 years old and space is rented )

I am not happy that they are going to bond to the steelwork with such a high difference between the MET and there. Why does the machine need a separate bond ? It is earthed through the swa. Has anyone else come across this ? I have searched the net and found lots of info regarding it from the USA but cannot find definite answers.

Any help advice is appreciated.

 

[telectrix]

i can't see any reason to bond the machine to the steel.if the steel is not tied down at 0v, then a fault down to the steel would also be transmitted to the machine. ask the guy what regulation he's using to request this.

 

[mort2376]

He doesn't know, it's just how they tell them to do it. It's a German machine, so maybe they have different regs ?

 

[telectrix]

german regs. are irrelevant in UK.

 

[Wilko]

Maybe the m/c bonding is from the dreaded "manufacturer's instruction" ? In terms of the steelwork itself, shouldn't it be bonded ? (Extraneous C.P. ?) Cheers, David.

 

[mort2376]

Maybe the m/c bonding is from the dreaded "manufacturer's instruction" ? In terms of the steelwork itself, shouldn't it be bonded ? (Extraneous C.P. ?) Cheers, David.

Yes the steelwork should be and I have pointed that out however as usual in a rented building its on the who pays for it list !

I am concerned about the potential difference between the main earth and the steel if they use it.

 

[westward10]

I don't see the point but if you do a main protective bond is essential.

 

[Carl S]

Bear in mind dust extraction and static electricity.

 

[Pat H]

If the structure is suitable bonded then cross bonding to it isn't safe. The SWA earth should be fine or run a separate earth direct to MET

 

[mort2376]

I have put it in writing that at present it is not safe in my opinion to bond to the structure due to no main bond to that. I have stated I will not connect it and have put the ball back in the installers court. Now the waiting game begins !

 

[Pat H]

Yes connect a red light between the structure and true earth. Then tell them if the red light comed on don't touch any part of the building. Maybe they will grasp the implications.

 

[Darkwood]

I have never seen a a cnc install where the structural is isolated from the machines earthed body, a pic would be interesting to see, also what is the earthing system for the building and is the structural steel within bridging distance of the machines supply earth?

 

[Pat H]

I'm assuming the structural mentioned is the building structure, not any additional "structure" to the machine. That I'd expect to be earthed as part of the install. Probably not possible to not be.

 

[mort2376]

It's the main building structure but it's a 100yr old mill and the space is rented so it's a who is going to pay for it situation. Not good but fairly typical I find.

The incoming is tns. The main transformer / substation is in the building and is the best place imho to bond the structural framework to as there are several boards throughout the building in different units.

 

[Pat H]

That makes sense but nit a nice can of worms to open. At that age there maybe issues with poor continuity across the whole structure. Could involve supplementary additional bonding to tie it all together.