isolators for earth bar or not?

I suspect they're not so much intended to be electrical isolators, so much as physical stand-offs - to give a bit of a gap behind the bar to make it easier to work (getting fingers around the back of the cables for example). On the other hand it possibly makes testing easier if you can remove parallel paths to the chassis easily.

   - Andy.

Thanks for reply. No, they actually do the job of electrical separators as well not only being a spacers.

We also use these bobbins/isolators for perimeter earth but it has a reason to test Zs loop impedance but why do they used in regular control panels...same reason? Don't thinks so.

I have seen it in the UK for systems with clear/dirty earth segregation. I suspect it is one of those things that is left in as it does no harm, and if you do need it, then it is already there. Saves stocking two sorts of spacer and a small risk of fitting the wrong one.. Then there are odd test and fault situations where you may prefer not to divert large currents through the box.
In those parts of the world that do permit customer side N-E links, it is often done with bars on posts this way to allow the electrode  linking to be tested in isolation of the supply N - which might confuse tests of the electrodes with current going into the electrodes next door.
Mike

Thanks. So if you install this kind of earth bars into control panel, would you use spacers or isolators?

That would depend where on the planet  it was going to be installed and the local earthing system. If that was unknown, I think the extra cost of providing insulating spacers and an optional link for the local installers to decide would be preferred to return for  re-work as  a 'covers all possible bases' option.
If  you know know for sure its TNS or TNCS with no local electrodes,  then you could fit metal.
Mike

There are TWO very good reasons why earth bars are installed on insulated mountings:

1. To better-control corrosion. Can be problematic for DC 'protective conductor currents' ('leakage currents') and also with certain frequencies.

2. For EMC purposes: the 'star point' earthing approach of BS IEC 61000-5-2 helps stop 'earth loops' which become important for higher frequency "noise". If you want the enclosure and/or backplate to be an EMC shield, it's better to individually connect each of the "panels" (front, back, left side, right side, top, bottom), each shelf/backplate, etc.,  through a copper braid or round conductor of between 6 and 16 sq mm. Similarly, with current-using equipment, if you connect cpc's back to the star point (vie the mains distribution units) in a separate way, rather than having "leakage currents" flowing around the cubicle itself, and also through any functional earthing, works best. Also, if CPFE (combined protective and functional earthing) is employed, the star point arrangement helps prevent PE currents flowing through purely FE connections, using the earth bar as the "earth reference" for that.

Correctly-applied star-point earthing in an overall common-bonding network can be particularly important for equipment in installations with inverter drives.

It would be important to take this into account, especially in the UK and EU countries, for conformity to the EMC DIrective (EMC Regulations 2016 in the UK).

On the EMC problems, it's worth noting that there is, in simple terms, a change over from star based earthing to screen/enclosure based 'earthing' around the point that the disturbance wavelengths roughly match the enclosure size.

Usually it's the point where stray capacitance (signal wire to 'earths' starts to form circuit loops and transmission lines.