Hammond transformer (simple question)

[leoparleur]

Hi,

Hammond does not mention were to hook up the grey wire on the 373HX. If used, and if it's the groung wire can I wire it to circuit ground or simply one of the 4 holding bolts will be ok?

http://www.hammondmfg.com/pdf/EDB373HX.pdf

Thanks,

[tatus]

The grey wire should be connected to the protective earch coming from your mains outlet. It is better to make a direct connection between the two inside the case, because the chassis may not create a safe electrical connection.

[AndrewT]

Nicely specified transformer. It does not look cheap.
The gry is connected to screen, the broken line running up and separating the primary from the secondary.
This reduces capacitive coupling of HF from mains to isolated circuits.
It is usual to connect this to chassis. That chassis must be connected permanently to PE.

[tatus]

It not only provides HF decoupling but also provides necessary safety "reinforced insulation". In case of an insulation fault of the primary, it prevents this fault from connecting to the secondary. This fault must safely be detected by your home installation's RCD device (residual current detector). For this reason there _must_ be a good electrical connection between that grey wire and PE.

[AndrewT]

Originally Posted by tatus

there _must_ be a good electrical connection between that grey wire and PE.

Yes, but it can connect to the chassis. It does not have to connect direct to the PE.
The Chassis must connect direct to the PE.

The leakage through to the screen is not intended to be detected by the RCCB (RCD). It is excess leakage that could nuisance trigger the RCCB.

Fault current flowing to the screen is detected by the MCB or on-board fuse, not by the RCCB.

[leoparleur]

First, thanks for all your comments! I just notice there aren't no gray wire to hook up. It wasn't apparent since it's already hooked up inside between the end bell and the core by the holding bolt.

For best continuity I guess i need to make sure the end bell leg needs to be to bare metal to have a good earth ground.

Thanks,

[leoparleur]

Originally Posted by AndrewT

Nicely specified transformer. It does not look cheap.
The gry is connected to screen, the broken line running up and separating the primary from the secondary.
This reduces capacitive coupling of HF from mains to isolated circuits.
It is usual to connect this to chassis. That chassis must be connected permanently to PE.

It was the only PT that provided the sufficient current to power my Shuguang EL156 tubes and 6SN7. I needed around 250ma, the 373HX provides 403ma. From Hammond, there is a big margin between this PT and closest ones (200ma).

Thanks,

[tatus]

@AndrewT: I think I have not expressed correctly what I meant. When I said the screen should be connected directly to PE I had in mind that there may not be a safe electrical connection (I mean safe in terms of regulatory directives) between different parts of the chassis. Consider a homemade anodized aluminium chassis or one made of power-coated steel. For such one you may have or have not good electrical connection between different parts. Connecting PE to one part of the chassis and connecting the transformer shield to another would result in an electrically unsafe device. If, on the other hand, the chassis is professionally manufactured, I think it is okay to assume that all parts are properly electrically connected, and the method that you propose is of course okay.

My second consideration was that the transformer's specification did not directly state whether the screen is a protective one or a functional one, and if it provides the required insulation level (reinforced) for use in unearthed equipment, or not. If it is using single insulation plus screen, then handling of the screen / PE connection must be done carefully.

Regarding fault current: depends on the impedance whether the RCCB or the MCB/fuse will trigger. A weak fault will be seen by the RCCB. Without the RCCB being in place, the device may already become hazardous to a person because the fault current may be much too low for the MCB, but already dangerous enough for an electric shock.