When do I need a protective earth?

Question

My company is developing a new handheld power tool. We are a rather inexperienced team and I am not a electrical engineer but I am concerned about the safety of our customers. We are located in Germany and while I am sure there are regulatory requirements, I feel like these are voluntarily (CE for example.) It seems like our device will be released without further safety tests and I am concerned. I am sorry about the non-technical nature of the question so far.

I made a sketch of our device:

The PCB pretty much converts 230 VAC to 230 VDC and drives a 230 VDC motor with a switch.

While most of the device is within a plastic case, there is a piece of metal sticking out of the front. The motor and the piece of metal at the front are pretty much connected to each other - I can measure only ~ 0.5 ohms between them. Our electrical engineer said there is no need for a protective earth because our competitors (3 rather large companies) don't have it either. Intuitively I would disagree since if there is a fault at the motor it would conduct the 230 VDC to the front piece of metal and touching that would be unfortunate.

Maybe I am just paranoid and not qualified so I hope someone can help me out here.

Answer 1

There's a lot of misconception around CE marking and other regulatory issues.

Of course, electrical safety is mandatory. All devices sold must be safe, i.e. state of the art. Luckily you don't have to find out for yourself, what's state of the art. You can rely to lots of rules which are issued by various regulatory bodies. First of all, DIN VDE rules cover the most important issues you have to take care of.

In your case the necessary air and creepage distances as well as isolation voltages have to be established for a class II device. It is possible to build a motor that is capable of handling all these requirements however I guess this motor will be expensive. If you just open any drilling machine with a plastic housing you will find that there's typically a single plastic cog wheel in the gearbox providing proper insulation between the motor and the touchable metal parts.

Unfortunately the demands on electrical safety vary between the markets you want to adress. There are different safety demands for home appliances, laboratory equipment, IT equipment and lighting fixtures, just to name the four most common. When you look up the EN standards you will find different air and creepage distances as well as different isolation voltages. For a new company it is a big task to build up the necessary knowledge about those standards. One of the biggest problems is, how to read them right and understand when to apply which standard.

So, as already mentioned in the comment section, you should consult experts and show them the whole device.

Answer 2

Things related to safety are described in standards. There is a LOT of them. They are not free and not always easy to read.

What I suggest, is to contact a Notified Body (institutions that issue CE certification) and ask for a "pre certification meeting" at your facility. In Germany, it could be the TÜV. You will have access to an expert for all the questions relative to the certification of your product. You will have an NDA in place with them, thus you can describe every single detail of your product to the expert along with the intended use/market of the product. They will tell you what norm is really relevant to you, what tests or certification measurement you have to conduct, etc...

It's less risky to ask all theses questions early on. You don't want to be in a situation where your development is finished and during the certification you learn that you have chosen the wrong kind of plastic that is not compatible with the fire safety. And that you have to redesign all your very expensive injection molds to accommodate for a new material...

Every time I have done a meeting with a notified body, I have significantly eased the certification process by modifying my design to carefully avoid any complex certification issues.

Of course everything has a cost. Certifying a product that includes electrical safety tests and EMC tests will end up to the ~10'000€ range...

Answer 3

You're in Germany to the UE standard applies. In particular for handheld tools you apply the Machinery Directive and in particular the Low Voltage Directive. You can look up the standards, sadly they are not free but the prices are reasonable if you buy them as EN standards (ask your DIN representative or online).

You can download for free the "Guide to the Application of the Machinery Directive" which is sort of a commented run-thru of the standard (and contains some of the text)

In short, but it needs a full risk analysis:

• Your metal part can become live in case of an insulation fault of the motor. The distance/clearance/creepage is not enough (in fact you've measured almost electrical continuity) so it is not safe;

• You can either do a protective earth bonding (Class I equipment, three pronged cord) to limit touch voltage in case of fault, or, redesign the tool with double insulation (Class II equipment, two pronged cord); details for both are in the directives and related standards;

• Your PCB needs to be safety reviewed too, if only because it could go in flames (it happens, trust me). There are peculiar circuit configurations to be aware of and you need to do a fault analysis for your primary-connected components (BTW this is way everybody prefer to buy precertified power supplies)

Your competitors most probably are doing class II which is quite popular in the hand held tool segment. Using a ground lead however is perfectly legitimate unless forbidden by some specific usage situation.

It's possible that you don't need to go to a notified body, the machinery directive only require it for specific types of machinery (the dreaded Annex IV list); check if it applies since a notified body costs a lot;

If you do not need to go to a notified body you can test much of the compliance yourself except for some specialized measurement like EMC compliance)

Don't forget also that electrical safety is not the only one, there are thermal, mechanical and chemical risks too (to say the most common ones).