Source 4 earth bond problem

[gsut1]

We've been having a spot of bother with the PAT testing of our MK1 Source 4's as a couple have failed the earth bond test. They've been reading 0.17 ohms on the tester against a pass threshold of 0.1 ohms. We then noticed that the earth bonding wire was getting really hot - hot enough to burn you if you touch it after testing.

 

Having got the units in bits we tested direct from the incoming earth cable and got a result of 0.04 ohms which would be a pass but when we connect the earth bonding strap directly to the incoming earth the test reading goes back up to 0.16 ohm - this implies to me that rather bizarrely the earth bond strap has to high a resistance.

 

Has anyone else come across this problem? We had a look at the MK2's and they all seem to have a slightly fatter copper cable rather than the steel one on the MK1's.

 

Very bizarre.

 

The odd thing is that in previous years these have tested out fine.

 

Thanks,

 

Graham.

[J Pearce]

Try cleaning the bonding point and crimp connections. Over time corrosion can build up and raise the resistance of the earth strap assembly.

This would tie in with only the older units being affected.

[gsut1]

Thanks for the advice but that was the first thing we tried. We took a file to the crimp ends and scraped the ridged washers about a bit. We then tested for continuity and it was fine but under load conditions the earth wire still heats up and gives a reading of 0.16 ohms. We've just taken some earth cable, stripped it, crimped it and used it instead and it's fine but this is less than ideal.

 

It seems to me that the original earth bond wire for the casing on the mk1's wasn't fit for purpose. Or has the test threshold been reduced for the earth bond test since they first came out?

[J Pearce]

The steel wire may be more prone to corrosion within the cable than the copper one.

 

I'm sure Tomo will be along with a definitive answer, as I suspect there'll be a reason for the change in bond between Mk1 and Mk2.

[grizzly]

Don't forget that the Earth Bond Test resistance should not exceed (0.1 + R) ohms, where R is the resistance of the protective conductor of the supply cord, or 0.1 ohms for appliances without a supply cord (IEE Code of Practice p42).

 

I can't remember off the top of my head if the Source Fours have a captive supply cord, or a detachable one, but either way, you should be taking the supply lead resistance into account during your Earth Bond test (if it is a detachable cord, this should be tested first, so you will know the cord EB resistance before you use it to test the luminaire).

There's a handy table of cable resistance values on p76 of the IEE CoP, and some newer PAT instruments can include the cable resistance during their test sequences.

 

Edited for spelling.

[Tomo]

We've been having a spot of bother with the PAT testing of our MK1 Source 4's as a couple have failed the earth bond test. They've been reading 0.17 ohms on the tester against a pass threshold of 0.1 ohms.

The reason for this is that you're testing them incorrectly.

 

(This gets a bit technical from here on...)

 

The PAT requirements are that equipment should be verified to still be within the safety limits prescribed in the original product standard tests that all equipment must undergo in order to be compliant for the CE Low Voltage Directive.

 

In the case of luminaires for stage lighting this means to standard EN60598 parts 1 and 17. The earth bond test in this standard requires an earth impedance of no more than 0.5 ohms, verified by passing a current of at least 10A. Nearly all common PAT’s will test at 25-30A and will show a fail for resistances above 0.1 ohm.

 

So in fact they do pass at 0.17 ohms.

 

We then noticed that the earth bonding wire was getting really hot - hot enough to burn you if you touch it after testing.

The stainless steel tether used to bond the lamp assy to the body is not able to pass 25A without self-heating to a considerable degree, 10A is the preferred test level for this reason. Note that this is perfectly legal and acceptable for compliance with the standard.

The odd thing is that in previous years these have tested out fine.

Actually, that's not odd.

The tether will be damaged by repeated testing above 10A - you've seen how hot it gets!

 

If you'd like a document to show your superiors, please get in touch with ETC Ltd and we'll be able to fax it to you.

 

The newer S4s use a copper bond wire to work around this misunderstanding.

[grizzly]

In the case of luminaires for stage lighting this means to standard EN60598 parts 1 and 17. The earth bond test in this standard requires an earth impedance of no more than 0.5 ohms, verified by passing a current of at least 10A. Nearly all common PAT’s will test at 25-30A and will show a fail for resistances above 0.1 ohm.

 

So in fact they do pass at 0.17 ohms.

 

This is quite interesting. I've had another flick through the IEE CoP, and BS EN 60598 is indeed listed in Appendix 1 (p49) in a list of "The following British Standards are relevant to equipment testing". However, at no point in the rest of the CoP (as far as I can see) is this higher earth bond test limit of 0.5 ohms mentioned, and certainly not in the section I mentioned above.

On p16 of the CoP, there is a small section on British and European standards, and it does say "Routine manufacturers' tests are not required for general in-service testing, but may be applied to new appliances or after repair."

 

I'd be interested to see if anyone can shed some light on this apparent discrepancy in the CoP.

[Brian]

The 0.1 figure is for items without a supply cord. For units with a supply cord you must factor in the appropriate figure.

 

Also, the 25A figure is a maximum not the recommended test current which is 1.5 times the rating of the fuse.

[grizzly]

The 0.1 figure is for items without a supply cord. For units with a supply cord you must factor in the appropriate figure.

 

Also, the 25A figure is a maximum not the recommended test current which is 1.5 times the rating of the fuse.

 

For units with a supply cord, the figure is (0.1 + R) ohms (R being the supply cord PC resistance), as mentioned in posts above. This is not the same as an earth bond pass limit of 0.5 ohms, as Tomo states is required by BS EN 60598. There is still a discrepancy here.

[Brian]

There is still a discrepancy here.

I see no discrepancy. The CoP states on page 8...

This guidance is limited to non-specialised situations such as offices, shops, hotels, schools and general industrial locations.

 

and on page 40...

...a person who is competent in the safe use of the test equipment and who knows how to interpret the results obtained.

...my bold.

 

In a specialised situation like a theatrical environment there should be no excuse for blindly following a CoP without understanding what you are doing. In this case you would be better off buying the industry specific CoP 'Code of Practice for Portable Appliance Testing & advice on electrical safety in theatres' published by the ABTT which says...

For stage luminaires the earth continuity from any metal part to the earth pin on the power connector must not exceed 0.5 Ohms...

[paulears]

I hope it doesn't say "luminaries", as that sounds like the Word spell check version, and wasn't proof read!

[Brian]

I hope it doesn't say "luminaries", as that sounds like the Word spell check version, and wasn't proof read!

Nope, that was me mistyping it.

[grizzly]

There is still a discrepancy here.

I see no discrepancy. The CoP states on page 8...

This guidance is limited to non-specialised situations such as offices, shops, hotels, schools and general industrial locations.

 

and on page 40...

...a person who is competent in the safe use of the test equipment and who knows how to interpret the results obtained.

...my bold.

 

In a specialised situation like a theatrical environment there should be no excuse for blindly following a CoP without understanding what you are doing. In this case you would be better off buying the industry specific CoP 'Code of Practice for Portable Appliance Testing & advice on electrical safety in theatres' published by the ABTT which says...

For stage luminaries the earth continuity from any metal part to the earth pin on the power connector must not exceed 0.5 Ohms...

 

 

There we go, then. That has exactly answered my question from a few posts ago. Thank you.

It's always nice if people are forthcoming when they know the answer, rather than posting too-brief answers that don't adequately address the query :)

 

Although... on page 8 of the IEE CoP, before the passage you quote, there is this: "The equipment within the scope of this document includes electrical appliances for household and similar use, certain IT equipment supplied by plug and socket, luminaires, and similar equipment."... my bold.

So it's not quite as clear cut as it would first appear.

[dbuckley]

In a specialised situation like a theatrical environment there should be no excuse for blindly following a CoP without understanding what you are doing. In this case you would be better off buying the industry specific CoP 'Code of Practice for Portable Appliance Testing & advice on electrical safety in theatres' published by the ABTT which says...

For stage luminaires the earth continuity from any metal part to the earth pin on the power connector must not exceed 0.5 Ohms...

 

Someone should remind the ABTT about Selecon profiles, for most metal parts of which this is not true...

[Tomo]

Someone should remind the ABTT about Selecon profiles, for most metal parts of which this is not true...

Actually, this also applies to the Source Four - the lens tube is not PE bonded and doesn't need to be - it can't become live unless the lamp house is live, and the lamp house is of course earth bonded.

 

I'll try to remember to pass that on to the ABTT - that comment could be misinterpreted.