Our engineering manager John Meijer shares his knowledge. Every month, he discusses a different topic in the field of electrical safety. This time his blog is about: grounding.
Control current transformer
A common mistake is that someone forgets to ground a control current transformer on the secondary side, which creates a floating network with all possible danger that could occur. NEN 60204-1 explains how to ground transformers. If you choose not to ground the transformer secondary, an earth fault relay should be used. The same applies for DC control power supplies.
Instrument earth rail coupling
In a control cabinet you often have a protective earth and a instrument earth rail. The instrument earth is insulated so that it does not connect to the mounting plate of the cabinet. This is mainly used to finish shielding cables from analogue signals or other sensitive (Ex i) equipment. It is often forgotten to connect this instrument earth rail to the main earth of the cabinet or installation. Then there will be a floating earth rail which causes even more misery.
If there is no separate instrument earth conductor, the insulated earth bus may be connected to the main earth bus via the shortest possible route with an earth conductor of min. 4mm2 or 2x 1.5mm2. This should be done as close as possible to the junction of the incoming protective earth of the switch box, unless there is too much contamination on it from other things coupled to the main earth busbar. In that case, a separate instrument earth connection must be installed.
The following topics were also discussed during John Meijer’s training sessions:
- What is the difference between a TT system, a TN system and a TNS system and how do I ground it?
- What is a protection chain and what are the requirements for construction?
- How do I determine the thicknesses of my earth conductors?
- How do I prevent stray currents through bearings with frequency-driven motors?
- When do I use an electrical circuit protection or earth leakage circuit breaker?
- How do I ground instruments and cables in an ATEX environment?
Do you have questions about above points? We can certainly advise you.