Safety requirements of a device

[hbot]

Hello everybody.

I'm developing a device, which has a metallic housing and a 3-phase connection that is intended only for measurements. The device is supplied from an external 24VDC source.
The device must fulfil the EN61010-1 safety norm, where is said, that "a device with a connection to mains must have a Protective Earth connection directly at the 3-phase terminals".

My question: if the housing of my device is metallic and already earthed and the 3-phase terminals are used only for measurements and carry no power, is it allowed not to do another one special PE-connection directly at the mains terminals? Please share your experience in such things.

 

[crutschow]

I have no direct experience with this, but the wording seems pretty clear. You must have an Earth connection to the 3-phase terminals, even if you have another ground connection to some other point. The reason for that is, if there were an accidental short from the 3-phase connection to the housing, you want a direct path for the current back to the 3-phase terminals. You don't want in wandering through the rest of your circuitry with possible serious consequences for the circuity and/or anyone in the vicinity. Whether the 3-phase connection carries power or not has nothing to do with it.

 

[hbot]

Don't understand you. Why does the current have to go through my circuit in the case of a phase short to the housing? It would flow via housing to the earth directly, or not? For such accidents we're using fuses in each phase.

 

[JimB]

Too many unknowns about your device.

Is it ever connected to the 3 phase supply when it is live?
If it is it will need a protective earth.

Or, is it just a piece of test equipment which is used when the 3 phase supply is isolated from the unit under test?
In this case, if you are just using a 24v supply to perform low voltage tests on the unit under test, I dont think a PE is required.

JimB

 

[hbot]

Too many unknowns about your device.

Is it ever connected to the 3 phase supply when it is live?
If it is it will need a protective earth.
Or, is it just a piece of test equipment which is used when the 3 phase supply is isolated from the unit under test?
In this case, if you are just using a 24v supply to perform low voltage tests on the unit under test, I dont think a PE is required.

JimB

The device is a kind of network analyser, which consists of two PC boards in a metallic housing. One board is called "analog board" and must be always connected to the mains with voltage up to 500Vmax. The board transforms the mains voltage to a lower value of 10V, using resistive dividers and OpAmps. Then this lower voltage is fed to a second board with ADCs and a microcontroller.

The device is always connected to the mains for the voltage measurements, but it is not supplied from there, but from an external isolated 24VDC-source.

The device has already a PE connection, it is implemented as a special screw terminal of the metallic housing. PC Boards have also a PE potential, that is used there for the filtering and protection purposes.

Firstly, the idea was just to connect the PE potential of the boards mechanically to the housing without any extra PE-connector on the boards themselves. But now we faced a safety standard (IEC/EN61010-1), that requires a special PE-connector very close to the 3 phase terminals. That's why this question appeared.

And I still don't understand, why just one connection of the housing to the PE is not enough?

 

[crutschow]

And I still don't understand, why just one connection of the housing to the PE is not enough?

If there is a short from the 3-phase line (before any fuse) to the housing, then you do not want that current (which could be large) flowing through some alternate path to ground. That alternate path may have a relatively high resistace to the 3-phase ground as compared to the 3-phase connection and thus generate dangerous voltages on the case. To avoid this possibility, the safety requirements mandate a direct ground connection to the 3-phase ground.

Is it that hard to provide the additional ground connection?

 

[hbot]

If there is a short from the 3-phase line (before any fuse) to the housing, then you do not want that current (which could be large) flowing through some alternate path to ground. That alternate path may have a relatively high resistace to the 3-phase ground as compared to the 3-phase connection and thus generate dangerous voltages on the case. To avoid this possibility, the safety requirements mandate a direct ground connection to the 3-phase ground.

Is it that hard to provide the additional ground connection?

It is possible. But why do we need to make two different PE-connections in one device?

Do you mean, that the PE-connection of the housing must be placed directly at the 3 phase terminals? Or one must use another extra PE-connector to the PC boards as in my situation?

 

[crutschow]

I would assume you need the 3-phase ground connection to the housing, since it's primarily a safety issue. Not sure if it also needs to be to the PC boards, if there's no safety issue there.

 

[mneary]

Does your 3-phase cable not have a protective earth? Does it meet code without a protective earth?

 

[hbot]

Does your 3-phase cable not have a protective earth?

The cable has no earth wire - usually we use three separate wires only for phases.

Does it meet code without a protective earth?

Other similar devices, that I've seen, haven't any protecitve earth wires in this cable either. But I'm not very sure about this. This also the reason why i asked my question here.

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As I understood from all your answers here, it's important to have a PE connection point of the housing as close to the 3-phase terminals as possible to reduce the current path from a phase to earth across the housing. I'm going to use this rule further.