EMC metal enclosure

[Kemme]

Hello all

Today I was playing with a device which had almost all of its pcbs covered in a metal enclosure. The designer explained me that the enclosure was added to protect the pcbs against the emc fields induced by the rest of the device. He also guaranteed that this metal case 100% fits the role.
But I am not sure. It had many holes in it and two big slits on two edges of the enclosure (like 15 cm long and 5mm in height). Picture below is just to visualize where the slit was located in the device.



My knowledge about emc is almost 0 but I have a feeling that this long slit completely negates all efforts with adding the case. Am I right?
All I could find on the internet was this:



SE stands for shielding effectiveness. Does that picture prove my point?

[Gyro]

Yes, you're correct. The maximum wavelength (minimum frequency) that an enclosure hole will pass is determined by it's longest dimension.

I notice that your enclosure picture shows two fixing screws along the length of the slit. As long as these joints are good and conductive, these will reduce the effective slit length. Whether you need to start looking at EMC gaskets etc. depends on the level and maximum frequency of emissions from your circuit. Remember also that conducted (and radiated) emissions from cables can be just as important.

[mike buba]

Sorry for raising the topic again, but it is a similar question.
I am looking for pcb metal enclosures, and all I can find are just metal boxes without any holes. I assume I have to drill holes (apertures) for power supply, ADC cables, GPIO, etc. Are there any limitations on how big/how many/distance between/etc.
If I look at simple Faraday cage design it is just a metal mesh.
Also, do I have to ground metal enclosure?