Ok I thought you had something more complete than this. It looks like you just have a table for each service type (electrical, hvac or plumbing) and a customer table and your service tables (plumbing and electrical) don't appear to be normalized. So let's be more generic, can you describe your contracting process?
So for instance if you have a customer with whom you contract for electrical services, what electrical services do you offer. The individual services (or service packages) need to be identified.
Let's say you have 10 electrical services
Code:
tblSvcType
ST_ID ST_Desc
1 Electrical
2 Plumbing
3 HVAC
tblSvc
SV_ID ST_ID SV_Description
1 1 Electrical Service 1
2 1 Electrical Service 2
3 1 Electrical Service 3
4 1 Electrical Service 4
5 1 Electrical Service 5
6 1 Electrical Service 6
7 1 Electrical Service 7
8 1 Electrical Service 8
9 1 Electrical Service 9
10 1 Electrical Service 10
11 2 Plumbing Service 1
Now let's say you have three 'packages' to offer on a contract package 1 obligates service 1 through 3, package 2 obligates 1 through 6 and package 3 obligates all of them you would have a junction table to indicate the 'packages' like
Code:
tblPackages
PA_ID ST_ID PA_Desc
1 1 Electrical Bronze Package
2 1 Electrical Silver Package
3 1 Electrical Gold Package
tblPackageSvc
PS_ID PA_ID SV_ID
1 1 1
2 1 3
3 1 2
4 2 1
5 2 2
6 2 3
7 2 4
8 2 5
9 2 6
10 3 1
11 3 2
12 3 3
13 3 4
14 3 5
15 3 6
16 3 7
17 3 8
18 3 9
19 3 10
So that when you went to build a contract you could choose the service package and know which particular services were supposed to be rendered. If you don't have 'packages' then you don't need this step.
From there when you build a schedule you would have to have some sort of mechanism for figuring service dates by the start date of the contract. i.e. if the contract is start date is january 2015 and the period is quarterly you could have your database build the schedule of creating 4 scheduled maintenance visits.
If electrical service 1 needs to be done once per quarter regardless of customer that would be set up on the services table, if electrical service 1 could be performed monthly, quarterly or annually, that would be on the scheduling side.
You can also do things like have the 'required' number of men for each service rendered so you can estimate how many people you should send to the job when the time comes. Or estimate the number of hours required by service so if you sum the hours of service required and you know how long you want to be at the site you can figure how many employees to send.
The payment/service rendered side would be another table that records who went on the job (individually rather than as a group would be my preferred method so you could track individual employee hours) and the number of hours they were there.
If you can explain more clearly what your business actually does (process wise, not service wise) in building a contract and delivering the contract that would be quite helpful.